deprecate hpc_load() and integrate it with restore()

[awaelchli]

What does this PR do?

Part of #7652

CheckpointConnector.hpc_load() does the same thing as CheckpointConnector(), except it additionally calls a hook
on_hpc_load on the module. This hook calling is now taken care of in restore().
Now we can properly deprecate the hpc_load for restore() to remove the code duplication.

The unit tests added here ensure that restore() behaves the same as the deprecated hpc_load!

Also closes #5371

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[codecov]

Codecov Report

Merging #7955 (017e36a) into master (20a5e09) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7955   +/-   ##
======================================
- Coverage      92%     91%   -0%     
======================================
  Files         203     203           
  Lines       13129   13125    -4     
======================================
- Hits        12017   11974   -43     
- Misses       1112    1151   +39     

[pep8speaks]

Hello @awaelchli! Thanks for updating this PR.

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

Comment last updated at 2021-06-14 11:54:07 UTC

[Borda]

I may use this wrapper but otherwise it looks food to me https://github.com/Borda/pyDeprecate#simple-function-forwarding