Updated check on model step output types from dict to Mapping

[nik777]

What does this PR do?

Fixes #17649

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📚 Documentation preview 📚: https://pytorch-lightning--18657.org.readthedocs.build/en/18657/

[awaelchli]

Solid! LGTM

[codecov]

Codecov Report

Merging #18657 (3d0f542) into master (f8e6fa4) will decrease coverage by 33%.
Report is 2 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18657      +/-   ##
==========================================
- Coverage      84%      50%     -33%     
==========================================
  Files         427      418       -9     
  Lines       33410    33235     -175     
==========================================
- Hits        27901    16770   -11131     
- Misses       5509    16465   +10956     

[carmocca]

I would still use "dict" in the error messages. Most people don't understand the difference between them.