UnitY implementation

[hirofumi0810]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• Did you read the contributor guideline?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

This PR supports new speech-to-speech translation models based on two-pass decoders.

• UnitY (text->unit)
• Translatotron2 (text->spectrogram)

These are related to this PR

• Reformat speech translation modules #4707
• Fix conformer encoder freezing #4721
• Fix eval mode for S2ST models during training #4722
• [WIP] Refactor sequence generator #4723
• Add Conv2dSubsampler #4724
• Add R-Drop #4730
• Support multi-task learning for speech-to-text task #4732
• Refactor S2ST #4747
• Support multitask for XMTransformer #4763
• Add a missing method to XMTransformer #4764

In addition to supporting new models, I also made several updates that may affect other tasks,

• Support dual cross-attention for Transformer decoder

Each of them is necessary to obtain the best result with UnitY.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃