Add MetricInputTransformer

[lgienapp]

What does this PR do?

Fixes #2371

This PR adds a new kind of metric wrapper for data transformations. Transformations allow for modifications to the input a metric receives by wrapping its pred and target arguments. Transformations can be implemented by either subclassing the MetricInputTransformer base class and overriding the .transform_pred() and/or .transform_target() functions, or by supplying a lambda function via the LambdaInputTransformer. A BinaryTargetTransformer which casts target labels to 0/1 given a threshold is provided for convenience.

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?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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📚 Documentation preview 📚: https://torchmetrics--2392.org.readthedocs.build/en/2392/

[codecov]

Codecov Report

Attention: Patch coverage is 38.77551% with 30 lines in your changes missing coverage. Please review.

Project coverage is 35%. Comparing base (4c2a143) to head (9c90ade).
Report is 101 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (4c2a143) and HEAD (9c90ade). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (4c2a143)	HEAD (9c90ade)
python3.8	3	2
cpu	20	19
macOS	5	4
torch1.13.1	1	0

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2392     +/-   ##
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- Coverage      69%     35%    -35%     
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  Files         313     314      +1     
  Lines       17615   17664     +49     
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- Hits        12161    6096   -6065     
- Misses       5454   11568   +6114     

[lgienapp]

Unsure what to do about the failing mypy test - assigning the lambdas to overwrite the original class methods seems to be the most straightforward solution to supplying user-defined behaviour here; I would add the file to [[tool.mypy.overrides]] to mute the error (since its the only one for transformations.py) unless someone sees a better in-code solution?

[Borda]

overall looks good to me :)

[baskrahmer]

Looks good :)

[Borda]

@lgienapp I just fixed some typing, but suddenly, all tests are failing, Do you mind checking what is wrong... 🦩

[lgienapp]

Some unit tests check for error messages via regexes, and these had to be adapted to the typo fixes as well. Should all pass now.

[stancld]

Nice work, thanks for the addition!