Prevent metric dtype conversion when metric is part of LightningModule

[FarzanT]

What does this PR do?

Fixes #1561

Added a simple check to prevent applying .half() and .float() calls that are passed on the LightningModule to metric states and defaults. Instead, users will have to use set_dtype(), as proposed in #493.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1583 (cfee14a) into master (0208d74) will decrease coverage by 0%.
The diff coverage is 38%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1583   +/-   ##
======================================
- Coverage      88%     88%   -0%     
======================================
  Files         225     225           
  Lines       11926   11938   +12     
======================================
+ Hits        10547   10555    +8     
- Misses       1379    1383    +4     

[Borda]

seems that the GPU tests are hanging... 🤔