Fix support for int input for samplewise classification

[SkafteNicki]

What does this PR do?

Fixes #1970
Missing dtype conversion to make sure that for multidim_average="samplewise" works with int input.

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.

Did you have fun?

Make sure you had fun coding 🙃

---

📚 Documentation preview 📚: https://torchmetrics--1977.org.readthedocs.build/en/1977/

[codecov]

Codecov Report

Merging #1977 (3256356) into master (b22ca97) will decrease coverage by 34%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1977     +/-   ##
========================================
- Coverage      85%     51%    -34%     
========================================
  Files         266     266             
  Lines       15183   15185      +2     
========================================
- Hits        12840    7687   -5153     
- Misses       2343    7498   +5155