Moved functions to pytorch_utils.py
# What does this PR do?

Moved all pruning stuff, apply_chunking_to_forward to pytorch_utils.py

Fixes #15543

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@patrickvonplaten @sgugger 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@patrickvonplaten I'm new to contributing to this repo. What other updates are required?<|||||>Those should be removed entirely honestly (components should be in model files, not modeling_utils), so I care little where they are :-)<|||||>@patrickvonplaten won't moving Conv1D back to modeling_utils.py cause circular imports?<|||||>@patrickvonplaten any thoughts?<|||||>@sgugger any more changes to complete on this PR?<|||||>Good to go for me!<|||||>Waiting for final review from @sgugger <|||||>I'm good on my side, was waiting for you Patrick ;-)