Add Musicgen
# What does this PR do?

Adds the musicgen model by fairseq to transformers

This model is made of three components:
1. T5Encoder (which import as `AutoModelForTextEncoding`)
2. MusicgenDecoder (which we copy as much as possible from [`modeling_bart.py`](https://github.com/huggingface/transformers/blob/1e9da2b0a6ef964c2cf72dd715dbee991a3f49fa/src/transformers/models/bart/modeling_bart.py#L142))
3. Encodec (which we import as `AutoModel`)

_The documentation is not available anymore as the PR was closed or merged._<|||||>Would be great to hear your thoughts on the design here @patrickvonplaten (adding the tests otherwise now)

TODO:
- [x] convert m/l checkpoints
- [x] handle padded tokens from encodec (in delay pattern mask, then again when we decode)
- [x] fast tests
- [x] integration tests
- [x] add method for unconditional generation (no need to use processor to get input ids)
- [x] finish docs / docstrings<|||||>This is ready for review (fyi @ArthurZucker / @patrickvonplaten) - kindly requesting review from @sgugger!