Fix FSDP v1 bug: trainer incorrectly uses an unwrapped model
Author: YanjunChen329
Pull request: yes

# What does this PR do?

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Fixes # (issue) https://github.com/huggingface/transformers/issues/39619

Currently, HG Trainer does not have the backward compatibility to support FSDP v1.

The bug happens at line 2378, which is a code path where both `delay_optimizer_creation` and `use_accelerator_prepare` are true. This seems to only be the case when FSDP v1 is used.

At line 2378, we set `self.model` to be the FSDPv1-wrapped model. However, at line 2403, we set `self.model` again to be `model`, which is the unwrapped model instance initialized at line 2361.

This bug makes the trainer use the unwrapped model in the following forward calls, which causes weight shape mismatch and uninitialization errors because all-gather calls in FSDP are not properly triggered.

This error can be fixed by replacing `self.model` with `model`, which makes it consistent with the other code paths


Example error:
[rank0]: RuntimeError: 'weight' must be 2-D


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