bugfix: torch.export failure caused by _make_causal_mask (#35291)

* bugfix: torch.export failure caused by `_make_causal_mask`

Recent changes in torch dynamo prevent mutations on tensors converted with aten::_to_copy. To address this, we can clone such tensor before performing in-place operation `masked_fill_` only when the code is being compiled by torch dynamo.
(relevant issue: pytorch/pytorch#127571)

* chore: use `is_torchdynamo_compiling` instead of `torch._dynamo.is_compiling`

src/transformers/modeling_attn_mask_utils.py

@@ -169,6 +169,10 @@ def _make_causal_mask(
             diagonal = past_key_values_length - sliding_window - 1
 
             context_mask = torch.tril(torch.ones_like(mask, dtype=torch.bool), diagonal=diagonal)
+            # Recent changes in PyTorch prevent mutations on tensors converted with aten::_to_copy
+            # See https://github.com/pytorch/pytorch/issues/127571
+            if is_torchdynamo_compiling():
+                mask = mask.clone()
             mask.masked_fill_(context_mask, torch.finfo(dtype).min)
 
         return mask[None, None, :, :].expand(bsz, 1, tgt_len, tgt_len + past_key_values_length)