BEST-RQ implementation

[whettenr]

What does this PR do?

Adds an implementation of BEST-RQ.
Add layer dropout interface for transformer classes.
Add output hidden layers and wrappers to be able to run MP3S benchmarks.

Fixes #<issue_number>

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[Adel-Moumen]

Hello @whettenr,

Thanks for this PR! We've recently merged in develop a lot of new PRs as part of SpeechBrain 1.0. Unfortunately, this PR as many conflicts due to the latests merge. Do you mind updating your fork and fix all the potential conflicts ? Thanks.

Best,
Adel

[whettenr]

Hey @Adel-Moumen,
Yes I will work on this! Thanks!
Ryan