[torch.fx] Abstract dynamic tracer
# What does this PR do?

We hope to abstract the dynamic features of tracer from the `HFTracer` class. The idea is to leverage the new feature for non HF modules, for example having a torch like tracer that support the dynamic allocation mechanisms.

If this PR is approved, we can move inheritance so that HFTracer implements DynamicTracer.


## 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/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@michaelbenayoun 

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Closing as @michaelbenayoun is making substantial changes in fx. Will create the abstraction again after his PR is merged.