Generate: TF supports multiple eos tokens
# What does this PR do?

TF generation test addition PR 4 (out of 4) 👉 All PT generation integration tests for features that also exist in TF are now framework-agnostic 🎉 

To complete this last step, I've added support to multiple EOS tokens in TF. 

Because this touches the core of the TF generation code, slow tests were run for:
- [x] TF GPT2
- [x] TF T5
_The documentation is not available anymore as the PR was closed or merged._<|||||>The two failing tests are known failures -- merging