TF: rework XLA generate tests
# What does this PR do?

In the light of recent findings (https://github.com/huggingface/transformers/issues/16838), this PR reworks existing XLA generate tests. The following key changes were made:
1. Added a `@unittest.skipIf` on XLA generate tests, to skip when no GPU is present;
2. Rework XLA `sample` tests -- due to the minor numerical differences that arise when we use XLA (and that we can't control), the sampling step will gather different samples even when we use the same seed. The only thing we can properly test is whether a) we can seed them and b) the results are sensible;
3. Adds at least one XLA test where the batch size is > 1 and the inputs have different lengths, so we can confirm that masking works (GPT-2 is not working 💔 , added a TODO);
4. Removes redundant tests (we had tests outside the integration tests that were testing the same thing).
_The documentation is not available anymore as the PR was closed or merged._<|||||>@patrickvonplaten reintroduced the fast tests, will merge as soon as CI gets to green