[bugfix] Add reloading support using BaseFinetuning

[tchaton]

What does this PR do?

This PR adds support for reloading the optimizer_state when training with BaseFinetuning.

Fixes #6891

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[codecov]

Codecov Report

Merging #7253 (bb9eb29) into master (969e857) will decrease coverage by 1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7253    +/-   ##
=======================================
- Coverage      91%     90%    -1%     
=======================================
  Files         199     199            
  Lines       12800   13014   +214     
=======================================
+ Hits        11704   11746    +42     
- Misses       1096    1268   +172     

[awaelchli]

the signature update could also have been in a separate PR, but LGTM

The only part I am not convinced about is passing the empty state to the hooks. This requires the user to change their implementation if I understand correctly. And it also requires us to update all our callbacks.

[ethanwharris]

LGTM, small query