[Bugfix] Fix location of unscale in mixed precision plugin

[justusschock]

What does this PR do?

Fixes #9599, #9330, #9205, #9694

Unscale was called after gradnorm tracking and gradient clipping. Moving it to a different hook in the plugins should fix that.

Alternative solution would be to move gradient tracking and clipping. Finding a solution that works for all plugin combinations might be difficult though.

Probably same purpose as #9287

Does your PR introduce any breaking changes? If yes, please list them.

None

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Please don't merge before review from @carmocca

[awaelchli]

@justusschock I think this is ready for review now

[codecov]

Codecov Report

Merging #9606 (225d5f4) into release/1.4.x (25bfd06) will decrease coverage by 4%.
The diff coverage is 45%.

@@              Coverage Diff               @@
##           release/1.4.x   #9606    +/-   ##
==============================================
- Coverage             93%     88%    -4%     
==============================================
  Files                218     218            
  Lines              14513   14519     +6     
==============================================
- Hits               13425   12814   -611     
- Misses              1088    1705   +617     

[awaelchli]

As discussed with @carmocca I rebased this onto the 1.4.x bugfix branch as we desire a better, less hacky long-term solution for master #9287 . So this is considered a hotfix.

There were basically no merge conflicts, the changes stay the same.

[tchaton]

LGTM !

[Hecim1984]

Review all