Add TF generate sample tests with all logit processors
# What does this PR do?

This PR adds a GPT2 TF generate sample test with all logit processors. It is a requirement for the TF generate sample refactor (https://github.com/huggingface/transformers/pull/15793), which is part of the TF generate refactor.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15852). All of your documentation changes will be reflected on that endpoint.<|||||>Also, code quality is failing, be sure to `make style`!<|||||>> Is the plan to add a single specific test for each logit processor, as they're converted?

Yeah, in the other PR (master doesn't have them yet)<|||||>Perfect! Sorry to be a bit annoying here - could you also add one random test for T5? I know that this is not a very common use case for encoder-decoder settings, but the encoder-decoder setting is significantly different from decoder-only so that it would be good to be sure that nothing is broken there before the refactor. Think you can take more or less the exact same input that you used for GPT2 (the output might not be that sensible, but anyways good to test encoder-decoder in sample mode).<|||||>@patrickvonplaten now with a T5 generate sample test. I took the liberty to find a combination of inputs where the output of one is very unstable (the first one), and the output of the other is pretty stable (the second one). It may help us distinguish the case where the new generate is completely different from the case where there are minor numerical differences.