CLI: tool to convert PT into TF weights and open hub PR
# What does this PR do?

This PR adds a CLI to convert PT weights into TF weights, validate them, and (optionally) open a PR. The open PR part depends on https://github.com/huggingface/huggingface_hub/pull/884, and is wrapped in a try/except for the time being.

Here are 3 PRs open with the tool (`transformers-cli pt-to-tf --model-name [model-name] --local-dir [local-dir] --open-pr`):
1. Text modality: https://huggingface.co/joaogante/test_text/discussions/1
2. Audio modality: https://huggingface.co/joaogante/test_audio/discussions/1
3. Image modality: https://huggingface.co/joaogante/test_img/discussions/1

This tool can also be used to check existing weights. Sadly, there is no programmatic way to check the weights in existing hub PRs, they have to be downloaded manually.
_The documentation is not available anymore as the PR was closed or merged._