Update cosine decay to not clamp output, match other implementations

[jlwitthuhn]

Proposed changes

This change updates how the 'minimum' value is treated by the cosine decay scheduler. The existing implementation clamps the output to the provided minimum and the new implementation instead smoothly transitions from the initial to final learning rate. This picture clarifies the difference between these two when decaying from 1.0 to 0.5 over 100 steps:

With this change the behavior will also match that of pytorch, optax, and keras

Also renamed 'minimum' to 'end' because this implementation can 'decay' upwards.

EDIT: I'm also not completely sold that removing minimum is the right move because it still might be useful to someone. Could be best to keep that and also add end as a separate parameter. If anyone has opinions about that I'd be happy to change it.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[awni]

Looks great! Thanks for fixing that.

I think it's fine to leave the minimum option out for now.