Generate: starcoder 🤜 🤛 assisted generation
# What does this PR do?

Starcoder (GPTBigCode) has a unique cache format, and assisted generation is heavy on cache-related ops. This PR adds the GPTBigCode special case.

All slow tests for assisted generation are passing after these changes.
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Arg. Would love to be able to change those parts directly in the model files in the future, instead of hacking special keys like this :-/

@sgugger I'm going to add a common test for the cache format, to ensure we don't do this again for future models :) 