iou_type: correct type hints

[adamjstewart]

What does this PR do?

Corrects a few issues with the type hints for iou_type:

• We sometimes use str and other times use Literal["bbox", "segm"]
• tuple[foo] means a tuple of length 1, use ... to make it a variadic tuple

Note that there are several other examples of the latter issue in the codebase, I only fixed iou_type.

Alternatively, we could force it to be a tuple of length 2.

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--2966.org.readthedocs.build/en/2966/

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 36%. Comparing base (dec80a5) to head (28b4362).

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2966     +/-   ##
========================================
- Coverage      69%     36%    -34%     
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  Files         332     332             
  Lines       19011   19011             
========================================
- Hits        13154    6773   -6381     
- Misses       5857   12238   +6381