[Feature] Support Perceptual Loss

[Taited]

Motivation

Perceptual Loss is an important and commonly used loss for improving perceptual quality in generation tasks. As discussed in issues 445, it is necessary to support perceptual loss in style transfer.

Modification

This PR makes modifications to files mmgeneration/mmgen/models/losses/__init__.py and mmgeneration/mmgen/models/losses/gen_auxiliary_loss.py. And there are unit tests in mmgeneration/tests/test_losses/test_gen_auxiliary_loss.py as well.

[codecov]

Codecov Report

Base: 74.68% // Head: 74.81% // Increases project coverage by +0.13% 🎉

Coverage data is based on head (54dd32a) compared to base (1869bcd).
Patch coverage: 89.10% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
+ Coverage   74.68%   74.81%   +0.13%     
==========================================
  Files         145      145              
  Lines       10914    11015     +101     
  Branches     2076     2097      +21     
==========================================
+ Hits         8151     8241      +90     
- Misses       2229     2233       +4     
- Partials      534      541       +7     

Flag	Coverage Δ
unittests	74.81% <89.10%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmgen/models/losses/__init__.py	100.00% <ø> (ø)
mmgen/models/losses/gen_auxiliary_loss.py	63.98% <89.10%> (+18.79%)	⬆️

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