TF: Correct XLA generation with GPT-2 and T5
# What does this PR do?

(WIP)
For future reference: the most recent commit, where T5 was adapted to discard padded past values (similarly to the FLAX implementation), works correctly numerically. However, when compiling to XLA, we get the following `NotImplementedError`:

<img width="1511" alt="Screenshot 2022-05-21 at 14 17 40" src="https://user-images.githubusercontent.com/12240844/169653445-25892c4b-3380-475b-ae24-861e840a9f05.png">


Because of that, I'll try a new strategy: the model receives as input the sliced past, without padding.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>The previous commit had a different approach (feed to the model the unpadded past), but resulted in the exact same exception. Both are related to a dynamic-size slicing of the past.<|||||>superceded by https://github.com/huggingface/transformers/pull/17426, which grabbed the good changes from this PR. T5 needs further 🔍 