Add TF benchmark examples
# What does this PR do?

As part of the objective to refresh TF examples, this one adds actual TF benchmarks in the examples. Also showcases the power of XLA ;)
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15853). All of your documentation changes will be reflected on that endpoint.<|||||>I see that this PR (https://github.com/huggingface/transformers/pull/15848) exists -- should I just delete the benchmark example instead?<|||||>Ah, and I didn't see that we're deprecating the benchmarks either. Maybe we shouldn't merge this!<|||||>Cc @patrickvonplaten <|||||>@patrickvonplaten It's inconsistent with deprecating the benchmarks, so is your approval a sign we should un-deprecate them?<|||||>That's fine by me! :-)<|||||>Cool -- I'm closing the PR then and will open a new one as @patrickvonplaten suggested (moving both benchmarks to the research projects).