Return the default_root_dir as the log_dir when the logger is a LoggerCollection

[gahdritz]

What does this PR do?

Fixes a bug whereby the PyTorch profiler would fail to write to a local file when the Trainer's logger is a LoggerCollection.

Fixes #8157

The way I see it, there are three ways to go about this:

1. Change the save_dir property of the LoggerCollection to the default_root_dir of the Trainer
2. What I did here
3. Some more complex fix, arguably the most natural, where the profiler traces are written to the save_dir of each constituent logger.

The first would probably break code that relies on the property being None to set defaults. The third, since the Lightning wrapper around the PyTorch profiler relies on PyTorch functions that expect a single path, is probably prohibitively complex.

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[pep8speaks]

Hello @gahdritz! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-07 00:26:02 UTC

[codecov]

Codecov Report

Merging #8187 (6941d93) into master (1afc1ca) will decrease coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #8187    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
=======================================
  Files         212     217     +5     
  Lines       13581   14263   +682     
=======================================
- Hits        12624   12577    -47     
- Misses        957    1686   +729     

[tchaton]

LGTM !