Fix mixed torch.Tensor and DTensor in generate when use FSDP2 + LoRA
Author: Xiao-Chenguang
Pull request: yes

# 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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Comments:
cc @3outeille maybe?

taking a look today

> lgtm but modify to this
> 
> ```python
> 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")
> ```

Updated

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_42436). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

ty for your work