fix amp/apex misconfiguration error for cpu

[awaelchli]

What does this PR do?

Fixes #6097

Apex/amp and CPU are not compatible.
The MisconfigurationError was never raised and tests are flawed, because they would run on ddp_cpu yet never get to the point where an error is raised because of early SystemExit...

More tests for accelerators will be added in #6102

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[codecov]

Codecov Report

Merging #6107 (c4e7c45) into master (3b0e4e0) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #6107   +/-   ##
======================================
  Coverage      93%     93%           
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  Files         160     160           
  Lines       11405   11405           
======================================
  Hits        10661   10661           
  Misses        744     744