[`PEFT`] Add HFTracer support for PEFT
# What does this PR do?

For more context, PiPPy is a library for an out-of-the-box Pipeline Parallelism for torch models. PiPPY heavily relies on HF tracer under the hood. 
Some interest has grown to support PiPPy for PEFT models: https://github.com/huggingface/peft/issues/194#issuecomment-1496767740 but it appeared that before this PR PEFT models were not supported by the HF tracer for many reasons. Therefore this PR addresses this, by doing precisely:

1- Relaxing the constraints for the model check in the tracing mechanism
2- Define a proper `__iter__` method for `HFProxy` class to properly handle `**kwargs` calls in the forward passes

A proper testing suite will be added in PEFT, as a set of slow tests, as the GH runner uses an environment that is complied with the main branch of transformers.

Thanks a lot @michaelbenayoun for digging the issue with me

cc @michaelbenayoun @sgugger @pacman100 

_The documentation is not available anymore as the PR was closed or merged._<|||||>I'm not against the auto-formatting changes, but could we have them in a separate PR please? This is polluting the diff here.