[BART] cleanup: remove redundant kwargs, improve docstrings
Small Bart code cleanups before pip release.

### Cleanup
- Deletes unused `value` argument for SelfAttention. (`value` is always the same as `key`.) This might be moderately controversial as most attention modules take query, key, value as arguments, but this change reduces the signature to just query and key (since key always the same as value).
- Deletes redundant `static_kv` argument for SelfAttention. It is always the same as `self.encoder_decoder_attention`. 
  - Context: the `static_kv` variable decides whether we want to extend the keys and values in the cache or, if `True`, use them without modification. 
  - This PR keeps a local `static_kv` variable because that variable name describes the purpose of the variable better than `self.encoder_decoder_attention`. But `static_kv` is no longer a kwarg. This simplifies the API and avoids having the same logic in two places.

#### Two new fast tests
- test coverage for `dummy_inputs` (previously broken)
- test coverage for the default generate kwargs.