Support output_scores in XLA TF generate
### Feature request

Support output_scores in XLA TF generate.

### Motivation

The scores would be critical and widely used in generation models for downstream applications. It is a confidence score for downstream thresholding or ranking. But it is not yet supported for XLA TF generate. 

### Your contribution

N/A
cc @gante @sgugger <|||||>Hey @rossbucky 👋 The support for returning scores with XLA is in our plans. Like other XLA-related changes, it requires rewriting that part of the code to work with static shapes, as opposed to lists of tensors that grow as generate runs. 

The next round of `generate` changes will focus on correctness (e.g. adding informative error messages). This feature will come right after that -- I'm keeping this issue open to track it publicly. <|||||>Hi, just curious if there is any update on this issue? Or, are there any known alternatives for easily getting confidence scores w/ XLA enabled, particularly for greedy decoding? It seems beam_search outputs a `sequences_score` when `return_dict_in_generate` is `True`, but don't see anything similar for greedy decoding.<|||||>No updates :) I have yet to build the appropriate piping to update and return those variables