FSDP grad accum fix
Author: winglian
Pull request: yes

# What does this PR do? 

https://github.com/huggingface/transformers/commit/8c62a92b3c3fea13652ef5d292a7863138410c59#diff-ed55888e6665791fe92cc8fc0c499da54f4ace6738551cd9a2591881cda076deL2473-R2480 seems to have broken FSDP training as we need to wrap the train step in no_sync on every step except the last step in the gradient accumulation batch. Without this, during the optimizer.step(), I get the error: 
```
RuntimeError: Tensors of the same index must be on the same device and the same dtype except `step` tensors that can be CPU and float32/64 notwithstanding
```

this wasn't caught by the fsdp tests since gradient accumulation wasn't part of those tests, so I also added tests for that as well.

see https://muellerzr.github.io/blog/gradient_accumulation.html#what-is-the-right-way-then

<img width="836" alt="Screenshot 2024-11-07 at 12 17 36 PM" src="https://github.com/user-attachments/assets/ceaef77e-5f87-45bb-8b72-c112fae3c2ee">

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Comments:
@ArthurZucker @muellerzr for review

is it possible to also trigger the slow tests for this PR too to make sure the new test passes?

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