Repository: huggingface/transformers
Pull request: Fix mixed torch.Tensor and DTensor in generate when use FSDP2 + LoRA

# What does this PR do?
`model.generate` raise an `Fix mixed torch.Tensor and DTensor` when FSDP2 and LoRA is applied.

The trainer class is the base class for trl trainers like PPOTrainer and GRPOTrainer in which `model.generate` is called during the training loop.
When FSDP2 is used, all gather and reshard is automatically done for `forward` method but not for `generate`.
This lead to the error above.

To fix it, there is a method `register_fsdp_forward_method` for torch to manage the DTensor for us.
By registering `generate`, we can get ride of the error.

generate is not properly supported under FSDP2 accelerating. This PR try to fix this error.
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Fixes #42417 (issue)


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Changed files:
filename: src/transformers/trainer.py
patch: @@ -2338,6 +2338,8 @@ def _inner_training_loop(
 
         if self.is_fsdp_enabled:
             self.model = self.model_wrapped = model
+            # Fix `got mixed torch.Tensor and DTensor` error in model.generate() for FSDP2 with LoRA
+            dist.fsdp.register_fsdp_forward_method(self.model, "generate")
 
         # for the rest of this function `model` is the outside model, whether it was wrapped or not
         if model is not self.model: