Generate: TF `compute_transition_scores`
# What does this PR do?

This PR adds the TF `compute_transition_scores`, akin to PT's #21191.

What seemingly started off as a simple task, ended up being a complex task -- the TF side had many missing and/or incorrect secondary `generate` outputs 😬 This means that we need to beef up the TF side of `generate` tests, which is much shorter than its PT counterpart.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@patrickvonplaten reverted the change in the order of operations in beam sample, and added a comment to based on our offline conversation.

LMK if you're happy with the PR 🙏 