[Feature] Support CPU training

[plyfager]

We support CPU training. Reference: open-mmlab/mmdetection#7016

[codecov]

Codecov Report

Merging #238 (5f022c0) into master (40376b4) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
- Coverage   76.75%   76.71%   -0.05%     
==========================================
  Files         130      130              
  Lines        9421     9421              
  Branches     1899     1899              
==========================================
- Hits         7231     7227       -4     
- Misses       1726     1728       +2     
- Partials      464      466       +2     

Flag	Coverage Δ
unittests	76.71% <0.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
mmgen/apis/train.py	20.00% <0.00%> (ø)
...chitectures/stylegan/generator_discriminator_v1.py	88.70% <0.00%> (-1.13%)	⬇️
mmgen/models/architectures/stylegan/mspie.py	90.59% <0.00%> (-1.00%)	⬇️

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