Generate: TF can now generate from embeddings in encoder-decoder models
# What does this PR do?

TF generation test addition PR 2 (out of ???).

In an effort to move generation integration tests to be framework-agnostic, I'll be adding low-hanging fruit to TF. This PR brings the ability to generate from input embeddings, with encoder-decoder models (or, more specifically, from non-input_ids inputs). The code added is almost copy-paste from PT.

Since this PR made a few changes in the main code path for generate, the following slow tests were run to ensure XLA compatibility:
- [x] GPT2
- [x] T5
_The documentation is not available anymore as the PR was closed or merged._<|||||>(merging -- the failing test, `test_from_pretrained_dynamic_model_distant` is a known flaky)