Clear predict_progress_bar in ProgressBar.__getstate__

[yifuwang]

What does this PR do?

Fixes #7567

tqdm is not pickle-able. ProgressBar.__getstate__() clears main_progress_bar, val_progress_bar, and test_progress_bar but somehow not predict_progress_bar. This PR addresses the issue.

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[justusschock]

Hi @yifuwang

thanks for your fix.

Could you try to get in some tests for that?

Feel free to take them over from #7609 (which I closed in favour of your PR) :)

[awaelchli]

good stuff!

[codecov]

Codecov Report

Merging #7608 (0e597f5) into master (20f6337) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7608    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         197     198     +1     
  Lines       12878   12932    +54     
=======================================
- Hits        11899   11370   -529     
- Misses        979    1562   +583     

[carmocca]

Added the test and updated the CHANGELOG