modeling_attn_mask_utils.py

# Copyright 2023 The HuggingFace Team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from dataclasses import dataclass
from typing import List, Optional, Tuple, Union

import torch


@dataclass
class AttentionMaskConverter:
    """
    A utility attention mask class that allows one to:
        - Create a causal 4d mask
        - Create a causal 4d mask with slided window
        - Convert a 2d attention mask (batch_size, query_length) to a 4d attention mask (batch_size, 1, query_length,
          key_value_length) that can be multiplied with attention scores

    Examples:

    ```python
    >>> import torch
    >>> from transformers.modeling_attn_mask_utils import AttentionMaskConverter

    >>> converter = AttentionMaskConverter(True)
    >>> converter.to_4d(torch.tensor([[0, 0, 0, 1, 1]]), 5, key_value_length=5, dtype=torch.float32)
    tensor([[[[-3.4028e+38, -3.4028e+38, -3.4028e+38, -3.4028e+38, -3.4028e+38],
            [-3.4028e+38, -3.4028e+38, -3.4028e+38, -3.4028e+38, -3.4028e+38],
            [-3.4028e+38, -3.4028e+38, -3.4028e+38, -3.4028e+38, -3.4028e+38],
            [-3.4028e+38, -3.4028e+38, -3.4028e+38,  0.0000e+00, -3.4028e+38],
            [-3.4028e+38, -3.4028e+38, -3.4028e+38,  0.0000e+00,  0.0000e+00]]]])
    ```

    Parameters:
        is_causal (`bool`):
            Whether the attention mask should be a uni-directional (causal) or bi-directional mask.

        sliding_window (`int`, *optional*):
            Optionally, the sliding window masks can be created if `sliding_window` is defined to a positive integer.
    """

    is_causal: bool
    sliding_window: int

    def __init__(self, is_causal: bool, sliding_window: Optional[int] = None):
        self.is_causal = is_causal
        self.sliding_window = sliding_window

        if self.sliding_window is not None and self.sliding_window <= 0:
            raise ValueError(
                f"Make sure that when passing `sliding_window` that its value is a strictly positive integer, not `{self.sliding_window}`"
            )

    def to_causal_4d(
        self,
        batch_size: int,
        query_length: int,
        key_value_length: int,
        dtype: torch.dtype,
        device: Union[torch.device, "str"] = "cpu",
    ) -> Optional[torch.Tensor]:
        """
        Creates a causal 4D mask of (bsz, head_dim=1, query_length, key_value_length) shape and adds large negative
        bias to upper right hand triangular matrix (causal mask).
        """
        if not self.is_causal:
            raise ValueError(f"Please use `to_causal_4d` only if {self.__class__} has `is_causal` set to True.")

        # If shape is not cached, create a new causal mask and cache it
        input_shape = (batch_size, query_length)
        past_key_values_length = key_value_length - query_length

        # create causal mask
        # [bsz, seq_len] -> [bsz, 1, tgt_seq_len, src_seq_len]
        causal_4d_mask = None
        if input_shape[-1] > 1 or self.sliding_window is not None:
            causal_4d_mask = self._make_causal_mask(
                input_shape,
                dtype,
                device=device,
                past_key_values_length=past_key_values_length,
                sliding_window=self.sliding_window,
            )

        return causal_4d_mask

    def to_4d(
        self,
        attention_mask_2d: torch.Tensor,
        query_length: int,
        dtype: torch.dtype,
        key_value_length: Optional[int] = None,
    ) -> torch.Tensor:
        """
        Converts 2D attention mask to 4D attention mask by expanding mask to (bsz, head_dim=1, query_length,
        key_value_length) shape and by adding a large negative bias to not-attended positions. If attention_mask is
        causal, a causal mask will be added.
        """
        input_shape = (attention_mask_2d.shape[0], query_length)

        # create causal mask
        # [bsz, seq_len] -> [bsz, 1, tgt_seq_len, src_seq_len]
        causal_4d_mask = None
        if (input_shape[-1] > 1 or self.sliding_window is not None) and self.is_causal:
            if key_value_length is None:
                raise ValueError(
                    "This attention mask converter is causal. Make sure to pass `key_value_length` to correctly create a causal mask."
                )

            past_key_values_length = key_value_length - query_length
            causal_4d_mask = self._make_causal_mask(
                input_shape,
                dtype,
                device=attention_mask_2d.device,
                past_key_values_length=past_key_values_length,
                sliding_window=self.sliding_window,
            )
        elif self.sliding_window is not None:
            raise NotImplementedError("Sliding window is currently only implemented for causal masking")

        # [bsz, seq_len] -> [bsz, 1, tgt_seq_len, src_seq_len]
        expanded_attn_mask = self._expand_mask(attention_mask_2d, dtype, tgt_len=input_shape[-1]).to(
            attention_mask_2d.device
        )

        if causal_4d_mask is not None:
            expanded_attn_mask = causal_4d_mask.masked_fill(expanded_attn_mask.bool(), torch.finfo(dtype).min)

        # expanded_attn_mask + causal_4d_mask can cause some overflow
        expanded_4d_mask = expanded_attn_mask

        return expanded_4d_mask

    @staticmethod
    def _make_causal_mask(
        input_ids_shape: torch.Size,
        dtype: torch.dtype,
        device: torch.device,
        past_key_values_length: int = 0,
        sliding_window: Optional[int] = None,
    ):
        """
        Make causal mask used for bi-directional self-attention.
        """
        bsz, tgt_len = input_ids_shape
        mask = torch.full((tgt_len, tgt_len), torch.finfo(dtype).min, device=device)
        mask_cond = torch.arange(mask.size(-1), device=device)
        mask.masked_fill_(mask_cond < (mask_cond + 1).view(mask.size(-1), 1), 0)

        mask = mask.to(dtype)

        if past_key_values_length > 0:
            mask = torch.cat([torch.zeros(tgt_len, past_key_values_length, dtype=dtype, device=device), mask], dim=-1)

        # add lower triangular sliding window mask if necessary
        if sliding_window is not None:
            diagonal = past_key_values_length - sliding_window + 1

            context_mask = 1 - torch.triu(torch.ones_like(mask, dtype=torch.int), diagonal=diagonal)
            mask.masked_fill_(context_mask.bool(), torch.finfo(dtype).min)

        return mask[None, None, :, :].expand(bsz, 1, tgt_len, tgt_len + past_key_values_length)

    @staticmethod
    def _expand_mask(mask: torch.Tensor, dtype: torch.dtype, tgt_len: Optional[int] = None):
        """
        Expands attention_mask from `[bsz, seq_len]` to `[bsz, 1, tgt_seq_len, src_seq_len]`.
        """
        bsz, src_len = mask.size()
        tgt_len = tgt_len if tgt_len is not None else src_len

        expanded_mask = mask[:, None, None, :].expand(bsz, 1, tgt_len, src_len).to(dtype)

        inverted_mask = 1.0 - expanded_mask

        return inverted_mask.masked_fill(inverted_mask.to(torch.bool), torch.finfo(dtype).min)

    @staticmethod
    def _unmask_unattended(
        expanded_mask: torch.Tensor, attention_mask: torch.Tensor, unmasked_value: Union[bool, float]
    ):
        # fmt: off
        """
        Attend to all tokens in masked rows from the expanded attention mask, for example the relevant first rows when
        using left padding. This is required by F.scaled_dot_product_attention memory-efficient attention path.
        Details: https://github.com/pytorch/pytorch/issues/110213

        `expanded_mask` is [bsz, num_masks, tgt_seq_len, src_seq_len] or [bsz, tgt_seq_len, src_seq_len].
        `attention_mask` is [bsz, src_seq_len].

        The dimension num_masks of `expanded_mask` is most often 1, but it can also be the number of heads in the case of alibi attention bias.

        For example, if `attention_mask` is
        ```
        [[0, 0, 1],
         [1, 1, 1],
         [0, 1, 1]]
        ```
        and `expanded_mask` is (e.g. here left-padding case)
        ```
        [[[[0, 0, 0],
           [0, 0, 0],
           [0, 0, 1]]],
         [[[1, 0, 0],
           [1, 1, 0],
           [1, 1, 1]]],
         [[[0, 0, 0],
           [0, 1, 0],
           [0, 1, 1]]]]
        ```
        then the modified `expanded_mask` will be
        ```
        [[[[1, 1, 1],   <-- modified
           [1, 1, 1],   <-- modified
           [0, 0, 1]]],
         [[[1, 0, 0],
           [1, 1, 0],
           [1, 1, 1]]],
         [[[1, 1, 1],   <-- modified
           [0, 1, 0],
           [0, 1, 1]]]]
        ```
        """
        # fmt: on

        # Get the index of the first non-zero value for every sample in the batch.
        # In the above example, indices = [[2], [0], [1]]]
        tmp = torch.arange(attention_mask.shape[1], 0, -1)
        indices = torch.argmax(attention_mask.cpu() * tmp, 1, keepdim=True)

        # Find the batch indexes that have unattended tokens on the leftmost side (e.g. [0, 0, 1, 1, 1]), for which the first rows of the
        # expanded mask will be completely unattended.
        left_masked_rows = torch.where(indices > 0)[0]

        if left_masked_rows.shape[0] == 0:
            return expanded_mask
        indices = indices[left_masked_rows]

        max_len = torch.max(indices)
        range_tensor = torch.arange(max_len).unsqueeze(0)
        range_tensor = range_tensor.repeat(indices.size(0), 1)

        # Avoid unmasking tokens at relevant target positions (on the row axis), by rather unmasking possibly several times the first row that should always be unmasked as we filtered out the batch above.
        range_tensor[range_tensor >= indices] = 0

        # TODO: we may drop support for 3D attention mask as the refactor from Patrick maybe dropped this case
        if expanded_mask.dim() == 4:
            num_masks = expanded_mask.shape[1]
            if num_masks == 1:
                # Broadcast [left_masked_rows, 1], [left_masked_rows, max_len]
                mask_slice = (left_masked_rows[:, None], 0, range_tensor)
            else:
                # Broadcast [left_masked_rows, 1, 1], [1, num_masks, 1], [left_masked_rows, 1, max_len]
                mask_slice = (
                    left_masked_rows[:, None, None],
                    torch.arange(num_masks)[None, :, None],
                    range_tensor[:, None, :],
                )
        else:
            # Broadcast [left_masked_rows, 1], [left_masked_rows, max_len]
            mask_slice = (left_masked_rows[:, None], range_tensor)

        expanded_mask[mask_slice] = unmasked_value

        return expanded_mask


def _prepare_4d_causal_attention_mask(
    attention_mask: Optional[torch.Tensor],
    input_shape: Union[torch.Size, Tuple, List],
    inputs_embeds: torch.Tensor,
    past_key_values_length: int,
    sliding_window: Optional[int] = None,
):
    """
    Creates a causal 4D mask of shape `(batch_size, 1, query_length, key_value_length)` from a 2D mask of shape
    `(batch_size, key_value_length)`

    Args:
        attention_mask (`torch.Tensor` or `None`):
            A 2D attention mask of shape `(batch_size, key_value_length)`
        input_shape (`tuple(int)` or `list(int)` or `torch.Size`):
            The input shape should be a tuple that defines `(batch_size, query_length)`.
        inputs_embeds (`torch.Tensor`):
            The embedded inputs as a torch Tensor.
        past_key_values_length (`int`):
            The length of the key value cache.
        sliding_window (`int`, *optional*):
            If the model uses windowed attention, a sliding window should be passed.
    """
    attn_mask_converter = AttentionMaskConverter(is_causal=True, sliding_window=sliding_window)

    key_value_length = input_shape[-1] + past_key_values_length

    # 4d mask is passed through the layers
    if attention_mask is not None and len(attention_mask.shape) == 2:
        attention_mask = attn_mask_converter.to_4d(
            attention_mask, input_shape[-1], key_value_length=key_value_length, dtype=inputs_embeds.dtype
        )
    elif attention_mask is not None and len(attention_mask.shape) == 4:
        expected_shape = (input_shape[0], 1, input_shape[1], key_value_length)
        if tuple(attention_mask.shape) != expected_shape:
            raise ValueError(
                f"Incorrect 4D attention_mask shape: {tuple(attention_mask.shape)}; expected: {expected_shape}."
            )
        else:
            # if the 4D mask has correct shape - invert it and fill with negative infinity
            inverted_mask = 1.0 - attention_mask
            attention_mask = inverted_mask.masked_fill(
                inverted_mask.to(torch.bool), torch.finfo(inputs_embeds.dtype).min
            )
    else:
        attention_mask = attn_mask_converter.to_causal_4d(
            input_shape[0], input_shape[-1], key_value_length, dtype=inputs_embeds.dtype, device=inputs_embeds.device
        )

    return attention_mask


# Adapted from _prepare_4d_causal_attention_mask
def _prepare_4d_causal_attention_mask_for_sdpa(
    attention_mask: Optional[torch.Tensor],
    input_shape: Union[torch.Size, Tuple, List],
    inputs_embeds: torch.Tensor,
    past_key_values_length: int,
    sliding_window: Optional[int] = None,
):
    """
    Prepares the correct `attn_mask` argument to be used by `torch.nn.functional.scaled_dot_product_attention`.

    In case no token is masked in the `attention_mask` argument, we simply set it to `None` for the cases `query_length == 1` and
    `key_value_length == query_length`, and rely instead on SDPA `is_causal` argument to use causal/non-causal masks,
    allowing to dispatch to the flash attention kernel (that can otherwise not be used if a custom `attn_mask` is passed).
    """
    attn_mask_converter = AttentionMaskConverter(is_causal=True, sliding_window=sliding_window)

    key_value_length = input_shape[-1] + past_key_values_length
    batch_size, query_length = input_shape

    # torch.jit.trace, symbolic_trace and torchdynamo with fullgraph=True are unable to capture the controlflow `is_causal=attention_mask is None and q_len > 1`
    # used as an SDPA argument. We keep compatibility with these tracing tools by always using SDPA's `attn_mask` argument in case we are tracing.
    # TODO: Fix this as well when using torchdynamo with fullgraph=True.
    is_tracing = torch.jit.is_tracing() or isinstance(inputs_embeds, torch.fx.Proxy)

    if attention_mask is not None:
        # 4d mask is passed through
        if len(attention_mask.shape) == 4:
            expected_shape = (input_shape[0], 1, input_shape[1], key_value_length)
            if tuple(attention_mask.shape) != expected_shape:
                raise ValueError(
                    f"Incorrect 4D attention_mask shape: {tuple(attention_mask.shape)}; expected: {expected_shape}."
                )
            else:
                # if the 4D mask has correct shape - invert it and fill with negative infinity
                inverted_mask = 1.0 - attention_mask.to(inputs_embeds.dtype)
                attention_mask = inverted_mask.masked_fill(
                    inverted_mask.to(torch.bool), torch.finfo(inputs_embeds.dtype).min
                )
                return attention_mask

        elif not is_tracing and torch.all(attention_mask == 1):
            if query_length == 1:
                # For query_length == 1, causal attention and bi-directional attention are the same.
                attention_mask = None
            elif key_value_length == query_length:
                attention_mask = None
            else:
                # Unfortunately, for query_length > 1 and key_value_length != query_length, we cannot generally ignore the attention mask, as SDPA causal mask generation
                # may be wrong. We will set `is_causal=False` in SDPA and rely on Transformers attention_mask instead, hence not setting it to None here.
                # Reference: https://github.com/pytorch/pytorch/issues/108108
                pass
    elif query_length > 1 and key_value_length != query_length:
        # See the comment above (https://github.com/pytorch/pytorch/issues/108108).
        # Ugly: we set it to True here to dispatch in the following controlflow to `to_causal_4d`.
        attention_mask = True
    elif is_tracing:
        raise ValueError(
            'Attention using SDPA can not be traced with torch.jit.trace when no attention_mask is provided. To solve this issue, please either load your model with the argument `attn_implementation="eager"` or pass an attention_mask input when tracing the model.'
        )

    if attention_mask is None:
        expanded_4d_mask = None
    elif attention_mask is True:
        expanded_4d_mask = attn_mask_converter.to_causal_4d(
            input_shape[0], input_shape[-1], key_value_length, dtype=inputs_embeds.dtype, device=inputs_embeds.device
        )
    else:
        expanded_4d_mask = attn_mask_converter.to_4d(
            attention_mask,
            input_shape[-1],
            dtype=inputs_embeds.dtype,
            key_value_length=key_value_length,
        )

        # From PyTorch 2.1 onwards, F.scaled_dot_product_attention with the memory-efficient attention backend
        # produces nans if sequences are completely unattended in the attention mask. Details: https://github.com/pytorch/pytorch/issues/110213
        #
        # This fix is not applied in case we are tracing with torch.jit.trace or symbolic_trace, as _unmask_unattended has a data-dependent
        # controlflow that can not be captured properly.
        # TODO: _unmask_unattended does not work either with torch.compile when using fullgraph=True. We should find a way to detect this case.
        if query_length > 1 and not is_tracing:
            expanded_4d_mask = AttentionMaskConverter._unmask_unattended(
                expanded_4d_mask, attention_mask, unmasked_value=0.0
            )

    return expanded_4d_mask


def _prepare_4d_attention_mask(mask: torch.Tensor, dtype: torch.dtype, tgt_len: Optional[int] = None):
    """
    Creates a non-causal 4D mask of shape `(batch_size, 1, query_length, key_value_length)` from a 2D mask of shape
    `(batch_size, key_value_length)`

    Args:
        mask (`torch.Tensor` or `None`):
            A 2D attention mask of shape `(batch_size, key_value_length)`
        dtype (`torch.dtype`):
            The torch dtype the created mask shall have.
        tgt_len (`int`):
            The target length or query length the created mask shall have.
    """
    return AttentionMaskConverter._expand_mask(mask=mask, dtype=dtype, tgt_len=tgt_len)


def _prepare_4d_attention_mask_for_sdpa(mask: torch.Tensor, dtype: torch.dtype, tgt_len: Optional[int] = None):
    """
    Creates a non-causal 4D mask of shape `(batch_size, 1, query_length, key_value_length)` from a 2D mask of shape
    `(batch_size, key_value_length)`

    Args:
        mask (`torch.Tensor` or `None`):
            A 2D attention mask of shape `(batch_size, key_value_length)`
        dtype (`torch.dtype`):
            The torch dtype the created mask shall have.
        tgt_len (`int`):
            The target length or query length the created mask shall have.
    """
    batch_size, key_value_length = mask.shape
    tgt_len = tgt_len if tgt_len is not None else key_value_length

    # torch.jit.trace and torchdynamo with fullgraph=True are unable to capture the controlflow `is_causal=attention_mask is None and q_len > 1`
    # used as an SDPA argument. We keep compatibility with these tracing tools by always using SDPA's `attn_mask` argument in case we are tracing.
    # TODO: Fix this as well when using torchdynamo with fullgraph=True.
    is_tracing = torch.jit.is_tracing()

    if torch.all(mask == 1):
        if is_tracing:
            pass
        elif tgt_len == 1:
            # For query_length == 1, causal attention and bi-directional attention are the same.
            return None
        elif key_value_length == tgt_len:
            return None
        else:
            # Unfortunately, for query_length > 1 and key_value_length != query_length, we can not generally ignore the attention mask, as SDPA causal mask generation
            # may be wrong. We will set is_causal=False in SDPA and rely on Transformers attention_mask instead, hence not setting it to None here.
            # Reference: https://github.com/pytorch/pytorch/issues/108108
            return AttentionMaskConverter._expand_mask(mask=mask, dtype=dtype, tgt_len=tgt_len)
    else:
        return AttentionMaskConverter._expand_mask(mask=mask, dtype=dtype, tgt_len=tgt_len)


def _create_4d_causal_attention_mask(
    input_shape: Union[torch.Size, Tuple, List],
    dtype: torch.dtype,
    device: torch.device,
    past_key_values_length: int = 0,
    sliding_window: Optional[int] = None,
) -> Optional[torch.Tensor]:
    """
    Creates a causal 4D mask of shape `(batch_size, 1, query_length, key_value_length)`

    Args:
        input_shape (`tuple(int)` or `list(int)` or `torch.Size`):
            The input shape should be a tuple that defines `(batch_size, query_length)`.
        dtype (`torch.dtype`):
            The torch dtype the created mask shall have.
        device (`int`):
            The torch device the created mask shall have.
        sliding_window (`int`, *optional*):
            If the model uses windowed attention, a sliding window should be passed.
    """
    attn_mask_converter = AttentionMaskConverter(is_causal=True, sliding_window=sliding_window)

    key_value_length = past_key_values_length + input_shape[-1]
    attention_mask = attn_mask_converter.to_causal_4d(
        input_shape[0], input_shape[-1], key_value_length, dtype=dtype, device=device
    )

    return attention_mask