[Feature] Add file mode for logger

[nbei]

Default implementation: https://docs.python.org/3/library/logging.handlers.html

[codecov]

Codecov Report

Merging #808 (d676fba) into master (5afca13) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #808   +/-   ##
=======================================
  Coverage   62.23%   62.23%           
=======================================
  Files         144      144           
  Lines        8506     8506           
  Branches     1522     1522           
=======================================
  Hits         5294     5294           
  Misses       2945     2945           
  Partials      267      267           

Flag	Coverage Δ
unittests	62.23% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmcv/utils/logging.py	100.00% <100.00%> (ø)

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