Update type hints for multiple dataloaders in .fit() & .test()

[awaelchli]

Before submitting

• Was this discussed/approved 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?
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Multiple dataloaders can be pased to fit and test.
The type hints should reflect that

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 🙃

[codecov]

Codecov Report

Merging #1723 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1723   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files          69      69           
  Lines        4131    4131           
======================================
  Hits         3655    3655           
  Misses        476     476           

[Borda]

/rebase