Fix visual progress bar bug / properly reset progress bar

[awaelchli]

What does this PR do?

Fixes #4567

A visual bug occurs at the very initial state of the progress bar:

Epoch 0:  33%|███▎      | 5/15 [00:05<00:10,  1.01s/it, loss=0.892, v_num=63]
Validating: 0it [00:00, ?it/s]

It shows "0 it" and the total is not displayed. But we can make it show just 0% like so:

Epoch 0:  33%|███▎      | 5/15 [00:05<00:10,  1.01s/it, loss=1.57, v_num=64] 
Validating:   0%|          | 0/10 [00:00<?, ?it/s]

by calling the tqdm.reset() method instead of manually setting the total amount.

This is very hard to spot, because the progress bar usually updates very fast. You have to add a time.sleep to the training loop to observe it.

[stale]

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you need further help see our docs: https://pytorch-lightning.readthedocs.io/en/latest/CONTRIBUTING.html#pull-request or ask the assistance of a core contributor here or on Slack. Thank you for your contributions.

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[codecov]

Codecov Report

Merging #4579 (fa80787) into master (d916973) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4579   +/-   ##
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  Coverage      93%     93%           
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  Files         135     135           
  Lines       10003   10006    +3     
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+ Hits         9326    9329    +3     
  Misses        677     677           

[Borda]

would be fine to merge it after #5495