remove deprecated args to learning rate step function

[SkafteNicki]

Before submitting

• [x ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x ] Did you read the contributor guideline?
• [x ] Did you make sure to update the docs?
• [x ] Did you write any new necessary tests?
• [x ] If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Fixes #708.

Removing of the deprecated epoch argument in the .step(...) method of learning rate schedulers. Currently (pytorch 1.4) if the user give a epoch argument, an DeprecationWarning will be thrown. But looking at the pytorch master branch, the warning has been changed to UserWarning meaning that in the future, if the epoch argument is not removed, this will give an warning each time a model with a learning rate scheduler is trained.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

[Borda]

LGTM 🚀