Improve check videos

[rlleshi]

There are some problems with the check_videos.py script. I didn't see any current PR or issues on it so made this one.
Also improved some minor stuff.

Bugs

• pool.join() was being called before pool.close(). It should be the other way around. This also leads to the following error:

Traceback (most recent call last):
  File "tools/check_videos.py", line 143, in <module>
    pool.join()
  File "/opt/conda/lib/python3.8/multiprocessing/pool.py", line 659, in join
    raise ValueError("Pool is still running")
ValueError: Pool is still running

• Getting file length
  The current way, len(f) doesn't work since objects don't have length. Corresponding error:

Traceback (most recent call last):
  File "tools/check_videos.py", line 148, in <module>
    print(f'Checked {len(dataset)} videos, '
TypeError: object of type '_io.TextIOWrapper' has no len()

Added a function to get file length.

• If no clips with problems were found then the script fails because no output file exists, e.g. here.
• Same thing as above for deleting corrupted clips when no output file exists

Minor improvements

• Remove num-processes as args and add instead all but one cpus for multiprocessing: pool = Pool(cpu_count() - 1 or 1)
• Add choices for args here and here

[codecov]

Codecov Report

Merging #950 (d78360c) into master (a6e8977) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head d78360c differs from pull request most recent head 68799d4. Consider uploading reports for the commit 68799d4 to get more accurate results

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
- Coverage   83.72%   83.68%   -0.05%     
==========================================
  Files         132      132              
  Lines        9983     9983              
  Branches     1722     1722              
==========================================
- Hits         8358     8354       -4     
- Misses       1210     1212       +2     
- Partials      415      417       +2     

Flag	Coverage Δ
unittests	83.68% <ø> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
mmaction/core/evaluation/accuracy.py	92.27% <0.00%> (-0.91%)	⬇️
mmaction/datasets/pipelines/augmentations.py	92.75% <0.00%> (-0.23%)	⬇️

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[dreamerlin]

I made some changes and now LGTM