Fix TQDM progress bar showing the wrong total when using a finite and iterable dataloader

[SkafteNicki]

What does this PR do?

Fixes #20695

Solves the case where multiple dataloaders are used, with the first being finite and the second being iterable (unknown total length). Due to these lines in tqdm:
https://github.com/tqdm/tqdm/blob/0ed5d7f18fa3153834cbac0aa57e8092b217cc16/tqdm/std.py#L1371-L1372
calling self.val_progress_bar.reset(total=None) after the first validation dataloader does not actually change the total value to None, thus resulting in the same total showing for the second dataloader. PR fixes this by explicitly setting the total attribute.

Before submitting

• Was this discussed/agreed 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 minor internal changes/refactors)

PR review

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

Reviewer checklist

• 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

---

📚 Documentation preview 📚: https://pytorch-lightning--21147.org.readthedocs.build/en/21147/