Fix number of issues in SpectralDistortionIndex

[SkafteNicki]

What does this PR do?

Fixes #1520

Three changes are taking place:

• Metric supports preds and target having different height/width. Removes restrictive check
• Corner case can lead to zero division. Adds eps to calculation
• Metric is slow due to double for-loop over channels, when calculated on many channels. Improves this by vectorizing the inner for loop. A quick timing of the old and new implementation showed that it is similar when number of channels are in the 1-3 range but the new implementation is up to 10x faster for 10 channels.

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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📚 Documentation preview 📚: https://torchmetrics--1808.org.readthedocs.build/en/1808/

[codecov]

Codecov Report

Merging #1808 (de02c9d) into master (962f82d) will decrease coverage by 44%.
The diff coverage is 49%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1808     +/-   ##
========================================
- Coverage      87%     43%    -44%     
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  Files         254     254             
  Lines       14278   14303     +25     
========================================
- Hits        12486    6180   -6306     
- Misses       1792    8123   +6331