Fix Prefix-Tuning for T5 models where d_kv != d_model / num_heads (#621)

Fixes #485.

Allows passing head dim (`n_embd_per_head`) explicitly to prefix tuning
to accomodate models where head dim is not equal to hidden dim /
n_heads.

src/adapters/methods/prefix_tuning.py

@@ -21,19 +21,20 @@ def __init__(
         n_heads: int,
         input_size: int,
         config: PrefixTuningConfig,
+        n_embd_per_head: Optional[int] = None,
     ):
         super().__init__()
         self.n_layers = n_layers
         self.n_heads = n_heads
         self.input_size = input_size
-        self.n_embd_per_head = self.input_size // self.n_heads
+        self.n_embd_per_head = n_embd_per_head or self.input_size // self.n_heads
         self.config = config
 
         self.wte = nn.Embedding(self.config.prefix_length, self.input_size)
         self.control_trans = nn.Sequential(
             nn.Linear(self.input_size, self.config.bottleneck_size),
             Activation_Function_Class(self.config.non_linearity.lower()),
-            nn.Linear(self.config.bottleneck_size, self.n_layers * 2 * self.input_size),
+            nn.Linear(self.config.bottleneck_size, self.n_layers * 2 * self.n_heads * self.n_embd_per_head),
         )
         self.dropout = nn.Dropout(self.config.dropout)
 
@@ -70,15 +71,18 @@ def __init__(
         n_heads: int,
         input_size: int,
         config: PrefixTuningConfig,
+        n_embd_per_head: Optional[int] = None,
     ):
         super().__init__()
         self.n_layers = n_layers
         self.n_heads = n_heads
         self.input_size = input_size
-        self.n_embd_per_head = self.input_size // self.n_heads
+        self.n_embd_per_head = n_embd_per_head or self.input_size // self.n_heads
         self.config = config
 
-        self.control_trans = nn.Parameter(torch.randn(self.config.prefix_length * self.n_layers * 2 * self.input_size))
+        self.control_trans = nn.Parameter(
+            torch.randn(self.config.prefix_length * self.n_layers * 2 * self.n_heads * self.n_embd_per_head)
+        )
 
         self.dropout = nn.Dropout(self.config.dropout)
 
@@ -174,6 +178,7 @@ def confirm_prefix(self, prefix_name: str) -> bool:
                 "n_layers": location_config["count"],
                 "n_heads": location_config["n_heads"],
                 "input_size": location_config["input_size"],
+                "n_embd_per_head": location_config["n_embd_per_head"],
             }
         prefix_tuning = PrefixTuningGroup(module_configs, prefix_tuning_config)
         prefix_tuning.train(self.training)  # make sure training mode is consistent
@@ -319,6 +324,7 @@ def add_adapter(self, adapter_name: str, layer_idx: int) -> bool:
                 self.location_key,
                 n_heads=self.model_config.num_attention_heads,
                 input_size=self.model_config.hidden_size,
+                n_embd_per_head=getattr(self.model_config, "d_kv", None),  # this is currently specific to T5-3B
             )
             self.prefixes[adapter_name] = prefix_id