Fix FSDPv2 checkpoint saving on TPU by using recursive unwrap
Author: Debashis2007
Pull request: yes
Labels: name: Code agent slop

When using FSDPv2 on TPU, models have nested FSDP wrappers around each transformer layer. The previous implementation only unwrapped the top-level wrapper, causing the saved checkpoint to contain wrapped state dict keys instead of the actual model parameters.

This fix uses unwrap_model with recursive=True specifically for FSDPv2 to unwrap all nested wrappers, then extracts the state dict from the fully unwrapped model. This ensures clean parameter keys in saved checkpoints while maintaining backward compatibility with FSDPv1 and other training configurations.

Fixes #36004

## What does this PR do?

Fixes checkpoint saving for FSDPv2 (SPMD) on TPU by properly unwrapping nested FSDP wrappers before extracting the model state dict.

When using FSDPv2 on TPU, models have nested FSDP wrappers around each transformer layer. The previous implementation only unwrapped the top-level wrapper, causing the saved checkpoint to contain wrapped state dict keys instead of the actual model parameters.

## Solution

Uses `unwrap_model` with `recursive=True` specifically for FSDPv2 to unwrap all nested wrappers, then extracts the state dict from the fully unwrapped model. This ensures clean parameter keys in saved checkpoints while maintaining backward compatibility.

## Fixes

Closes #36004
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