[1 / 3] improvements to saving and loading callback state

[awaelchli]

What does this PR do?

Initial PR for an improved state saving and loading for Callbacks.

1. This is a bugfix, since pickling with the keys type(callback) would pickle the callback class source, and this prevents loading the checkpoint outside of PL (when PL not installed).

2. Feature: With a state identifier, we will be able to produce a (more) unique key for the callbacks. It allows us to save more than one of the same type of callback since we can now disambiguate. This PR does not yet add unique identifiers. This will come in a follow up part.

This change is fully backward compatible (old checkpoints can still be loaded the same way).

Part of #6467

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Did you have fun?

I made sure I had fun coding 🙃

[pep8speaks]

Hello @awaelchli! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-28 21:31:21 UTC

[codecov]

Codecov Report

Merging #6886 (302e724) into master (aadd2a9) will increase coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #6886    +/-   ##
=======================================
  Coverage      92%     93%            
=======================================
  Files         218     169    -49     
  Lines       14407   13959   -448     
=======================================
- Hits        13305   12917   -388     
+ Misses       1102    1042    -60