Complete test for batch size < num gpus

[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?

Follow up to #1609
Uses the new TemplateModel for testing.

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 #1705 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1705   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files          69      69           
  Lines        4151    4151           
======================================
  Hits         3661    3661           
  Misses        490     490           

[mergify]

This pull request is now in conflict... :(