[Speech Model] Add Emformer
# What does this PR do?

This PR adds the Emformer model: an auto-regressive ASR model with an option for RNN-Transducer decoding.
This model shows promising result for real-time speech recognition and there are 3 pretrained checkpoints available via the torchaudio library:
https://pytorch.org/audio/main/tutorials/online_asr_tutorial.html

Original `torchaudio` implementation: https://github.com/pytorch/audio/blob/main/torchaudio/models/emformer.py

Paper: https://arxiv.org/abs/2010.10759

RNN-Transducer details for reference: https://lorenlugosch.github.io/posts/2020/11/transducer/


@anton-l could you leave some comments in the code so that I know where I should take a look for the model design here?<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17302). All of your documentation changes will be reflected on that endpoint.<|||||>@anton-l do you want to fix the failling tests or should I review already?<|||||>@patrickvonplaten I've left a comment about the last major failing test above, but the rest is ready for review :) 
Also I'm not sure what happened to the sentencepiece tests here https://app.circleci.com/pipelines/github/huggingface/transformers/41150/workflows/91d3a0c9-f633-4895-9169-3179f759ced6/jobs/468492, could you take a look please?<|||||>> @patrickvonplaten I've left a comment about the last major failing test above, but the rest is ready for review :) Also I'm not sure what happened to the sentencepiece tests here https://app.circleci.com/pipelines/github/huggingface/transformers/41150/workflows/91d3a0c9-f633-4895-9169-3179f759ced6/jobs/468492, could you take a look please?

See: https://huggingface.slack.com/archives/C01NE71C4F7/p1653573450540219?thread_ts=1653570610.579129&cid=C01NE71C4F7 

Rebase to main should solve the issue<|||||>@patrickvonplaten looks like the remaining failed tests are unrelated <|||||>Taking this PR over! <|||||>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.<|||||>Actually closing this for now as I'm not sure anymore whether this should go into `main`. We should maybe eventually think about a new speech library on top of Transformers<|||||>If anyone from the community is interested in taking over this PR, please feel free to do so!