Template for framework-agnostic tests
# What does this PR do?

There are a few cross-framework pain points I often encounter: 
1. Ensuring the interface of `.generate()` or models stay consistent across frameworks;
2. Ensuring that TF doesn't get neglected and satisfies 1., when contributors add features/fixes on the PT side;
3. Blocking cases where the interface is the same, but there are numerical differences.

After a brief chat with @sgugger, I thought inheritable framework-agnostic tests could be nice to help with these problems. This week I'll have to write a few TF `.generate()` tests that already exist on the PT side, so this could be a great chance to kill 2 birds with one stone. 

However, I'd like to get the pattern right, hence this small PR -- replaces a pair of framework-specific tests with a framework-agnostic test. `Flax` is intentionally left out, as it is missing many testable features and is not being maintained, but it can easily be added in the future.

Let me know what you think of it!
_The documentation is not available anymore as the PR was closed or merged._<|||||>> I might be missing something, but not clear to me how/where this is tested here.

@amyeroberts I see, this example does not test numerical differences :) 

However, consider the tests [here](https://github.com/huggingface/transformers/blob/main/tests/generation/test_logits_process.py), which also exist for TF. They are primarily numerical tests, they test the resulting vector against a constant. It is not uncommon for a user to find an edge case on these processors, fix the PT side of it, and add a few more numerical checks. These new numerical checks would fail in TF, but I often can't convince the users to make the change on the TF side (and then fail to follow up myself after it is merged). Result: a bug that is trivial to fix in that moment gets lost 🙈 This unified framework would prevent it -- the fix would need to include the corresponding TF change to pass the tests (either fixed by the user or by one of us). PT == expected values == TF