Add note for expectation of custom feature extrator in FID metric

[SkafteNicki]

What does this PR do?

Fixes #2266

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2277.org.readthedocs.build/en/2277/

[codecov]

Codecov Report

Merging #2277 (5765495) into master (4059085) will decrease coverage by 46%.
The diff coverage is 0%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2277     +/-   ##
========================================
- Coverage      87%     41%    -46%     
========================================
  Files         300     300             
  Lines       16969   16971      +2     
========================================
- Hits        14737    6970   -7767     
- Misses       2232   10001   +7769