Add support of backward_prefetch and forward_prefetch
#21156 

Adds support for backward_prefetch and forward_prefetch in trainer.

@sgugger @pacman100 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger Done, But not sure why this test is failing. Any pointers on how to make this build green would help.<|||||>@sgugger @pacman100  Need pointer on why this test is failing.<|||||>> The test is a flaky one, don't worry about it. Thanks for iterating, I just have one last comment on the deprecation warning for `fsdp_min_num_params` and we can merge this!

Done
<|||||>@sgugger @pacman100  Can we merge
 this PR ? <|||||>?Hello @raghavanone , could you please resolve the comments above that I have unresolved as they are yet to be addressed ?<|||||>> ?Hello @raghavanone , could you please resolve the comments above that I have unresolved as they are yet to be addressed ?

Done<|||||>Thank you @raghavanone for iterating and addressing the comments and for the overall contribution! 🚀 