Musicgen: CFG is manually added 
# What does this PR do?

This PR exists to keep `musicgen`'s current functionalities considering the changes in #24654.

In a nutshell, the #24654 has a more flexible version of CFG (allows negative prompting, is compatible with existing generation methods, doesn't need to expand the batch size by 2 before the fwd pass, lower memory requirements), but would mean an execution time regression on `musicgen` (because it needs 2x forward passes).
@sanchit-gandhi there is a potential change due to order of operations, depending on the processors commonly used. Would you be able to confirm whether it would be okay like this?<|||||>_The documentation is not available anymore as the PR was closed or merged._