Rename callbacks/base.py to callbacks/callback.py

[daniellepintz]

Proposed refactor

Rename callbacks/base.py to callbacks/callback.py

Motivation

For consistency, let's conform to the standard of our other main files, like accelerators/accelerator.py and strategies/strategy.py.

Part of #11908

cc @Borda @justusschock @awaelchli @rohitgr7 @ananthsub @daniellepintz

[PushanAgrawal]

Hello, can I make a pull request for this issue?

[daniellepintz]

Yes! Assigning you :)

[shantam-8]

Hello @daniellepintz, is this issue still open? If yes, should I make a new PR?

[akihironitta]

Hi @PushanAgrawal! Thank you for working on this in #11996! Would you like to continue the linked PR or let another contributor work on this issue instead?

Hi @shantam-8! Thanks for your interest in contributing! Let's wait maybe for a few days to hear if @PushanAgrawal still wishes to continue to work on it :] In the meantime, you could pick one of the issues labeled "good first issue" or "help wanted"! (#12521 has a lot of TODO items and definitely needs help from the community!)

[akihironitta]

@shantam-8 Would you be interested in submitting a PR?