deduplicate saved_for_backward

thunder/core/trace_interpreter.py

@@ -213,7 +213,6 @@ def __init__(self, trace, *args, **kwargs):
         self.trace = trace
         self.new_trace = from_trace(self.trace)
         self.have_processed_args = False
-        print(self.trace)
 
     def read(self, x: VariableInterface | Any) -> Any:
         if isinstance(x, VariableInterface):

thunder/core/vjp_utils.py

@@ -1,5 +1,5 @@
 import inspect
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from functools import wraps
 from inspect import Parameter, Signature
 from itertools import chain
@@ -229,3 +229,22 @@ def get_saved_for_backward_tensors(trace: TraceCtx) -> tuple[TensorProxy]:
         lambda: "All saved tensors must be TensorProxy or None",
     )
     return tuple(saved_tensors)
+
+
+def set_saved_for_backward_tensors(trace: TraceCtx, saved_tensors: Sequence[TensorProxy]):
+    """
+    Given a trace, return the tensors that are saved for backward in the trace.
+
+    Args:
+        trace: The trace to set saved tensors for.
+        saved_tensors: proxies for the tensors to save.
+    """
+    utils.check(
+        all(isinstance(t, TensorProxy) or t is None for t in saved_tensors),
+        lambda: "All saved tensors must be TensorProxy or None",
+    )
+    ret_node = trace.bound_symbols.pop(-1)
+    assert ret_node.sym == prims.python_return
+    output = ret_node.args
+    output = (output[0], (tuple(saved_tensors), *output[1][1:]), *output[2:])
+    trace.bound_symbols.append(ret_node.from_bsym(args=output))

thunder/executors/torch_autograd.py

@@ -10,7 +10,7 @@
 from thunder.core.symbol import BoundSymbol
 from thunder.core.trace import TraceCtx, from_trace, set_tracectx, reset_tracectx
 from thunder.core.transform_common import replace_redundant_inputs
-from thunder.core.vjp_utils import get_saved_for_backward_tensors
+from thunder.core.vjp_utils import get_saved_for_backward_tensors, set_saved_for_backward_tensors
 
 if TYPE_CHECKING:
     from thunder.core.trace import VariableInterface
@@ -240,6 +240,23 @@ def split_forward_backward(computation_trc: TraceCtx, compile_data, compile_stat
         skip_output=False,
         skip_subsymbols=False,
     )
+
+    # remove duplicates
+    # The NVFuser (and possibly others) fusion pass applied on the forward during has a
+    # CSE pass that may lead to duplicate symbols saved for backward. This causes trouble
+    # because we see duplicates in the unpacking. But the passes are unaware of the backward,
+    # so they cannot handle it themselves, so we clean this up here.
+    seen = set()
+    new_fw_out = []
+    new_bw_inp = []
+    for p_fw, p_bw in zip(get_saved_for_backward_tensors(fw_extrace), new_bsyms[4].output, strict=True):
+        if p_fw.name not in seen:
+            seen.add(p_fw.name)
+            new_fw_out.append(p_fw)
+            new_bw_inp.append(p_bw)
+    new_bsyms[4] = new_bsyms[4].from_bsym(output=tuple(new_bw_inp))
+    set_saved_for_backward_tensors(fw_extrace, new_fw_out)
+
     bw_trace.bound_symbols = new_bsyms
 
     if getattr(compile_data.fn, "use_fsdp", False):