Framework split
# What does this PR do?

This PR prepares the Transformers repo for the new syntax for framework-specific content. It should be merged once https://github.com/huggingface/doc-builder/pull/130 and https://github.com/huggingface/doc-builder/pull/63 are merged.

Note to @stevhliu : This only concerns existing code with switches, not text (except in the cases where the text dependent on PT vs TF was just before the code). It's probably worth doing a pass on the code to see what paragraphs could be split in framework specific blocks once this PR is merged. For instance, all task tutorials could have the whole fine-tuning in two such blocks.
_The documentation is not available anymore as the PR was closed or merged._<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16030). All of your documentation changes will be reflected on that endpoint.