issues in `generate()`
## A collection of issues for `generate()` method that we need to address

(**I don't assign these issues to anyone yet. I might work on some of these later. The purpose here is to make these issues transparent to the community, and to serve as a TODO list so we won't forget.**)

As an extension to the model equivalence tests across framework, I prepared a list of potential issues for `generate()` which gives different results across frameworks.

- Currently only look at the type and shape, not the values inside the tensors
- We might need to wait the current **`generate()` refactorization PR(s) merged** before starting address these issues.
- Here is the [Colab notebook](
https://colab.research.google.com/drive/1294oyVDrwvnuw_P3QtFE_CZ7DoMdthyq?usp=sharing) which demonstrates the issues.

Summary of issues (also included as comments in the above notebook)

- Flax's `generate()` doesn't support `return_dict_in_generate` (gives errors), `output_scores`, `output_attentions`, while PT/TF accept them
- PT/TF/Flax `BeamSearchEncoderDecoderOutput` outputs have different keys
- `scores` have different types: PT -> `tuple` , TF/Flax -> `tensor` 
- `scores` have different shape: PT -> (15, 4, 50257), TF -> (1, 4, 50257), Flax -> (1,)
- `sequences_scores` have different types: PT -> tensor , TF -> None , Flax -> no such attribute 
- `PT.sequences_scores` seems to be `Flax.scores`
- TF's `sequences` has a shorter length by 1 than PT's `sequences` 
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.