fix pesq docs

[pme0]

What does this PR do?

Fixes #2095

PESQ (torchmetrics.audio.pesq.PerceptualEvaluationSpeechQuality) docs incorrectly state that a tensor of pesq values is returned. This PR:

• corrects this inconsistency between docs and implementation, namely that forward and compute return metric value reduced across the batch, instead of a tensor of metrics values for each sample;
• adds a note guiding the user to the functional version of PESQ (torchmetrics.functional.audio.pesq.perceptual_evaluation_speech_quality) if a tensor of metrics values for each sample is required;

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.
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📚 Documentation preview 📚: https://torchmetrics--2114.org.readthedocs.build/en/2114/

[codecov]

Codecov Report

Merging #2114 (f2fef22) into master (1ddacab) will decrease coverage by 51%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2114     +/-   ##
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- Coverage      87%     36%    -51%     
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  Files         293     293             
  Lines       16299   16299             
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- Hits        14177    5837   -8340     
- Misses       2122   10462   +8340