[loops] Reset reference to dataloader iterator on run end

[ananthsub]

What does this PR do?

This cleans up the dataloader_iter inside of the training epoch and evaluation epoch loops. The dataloader iterator is initially set in on_run_start which indicates we should have this cleanup in on_run_end to avoid any dangling references around.

Does your PR introduce any breaking changes? If yes, please list them.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #9386 (cd766b8) into master (41ba639) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9386    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         179     179            
  Lines       14905   14907     +2     
=======================================
- Hits        13751   13140   -611     
- Misses       1154    1767   +613     

[tchaton]

LGTM !