Use fsspec with OmegaConf saving in saving.py

[ananthsub]

What does this PR do?

Currently, the config_yaml is restricted to be saved to a local filepath when using an OmegaConf config. However, Omegaconf.save accepts an IO object. Opening the file and passing the filepointer to OmegaConf enables Lightning to save the config yaml to any storage backends enabled by fsspec.

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[Borda]

lgtm

[codecov]

Codecov Report

Merging #3782 into master will increase coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #3782    +/-   ##
=======================================
+ Coverage      85%     90%    +5%     
=======================================
  Files         110     110            
  Lines        8539    8685   +146     
=======================================
+ Hits         7295    7844   +549     
+ Misses       1244     841   -403     

[awaelchli]

Thanks Ananth! 🚀