[Feature] Exponential Moving Average Hook

[ckkelvinchan]

Motivation

Many existing methods, such as Real-ESRGAN, use exponential moving average hook during training.

Modification

This PR supports exponential moving average hook. Users can define this hook in the configuration file.

Remark

Codes adopted from MMGeneration.

[codecov]

Codecov Report

Merging #542 (df46f0a) into master (7037144) will increase coverage by 0.05%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
+ Coverage   80.28%   80.34%   +0.05%     
==========================================
  Files         194      195       +1     
  Lines       10805    10857      +52     
  Branches     1640     1650      +10     
==========================================
+ Hits         8675     8723      +48     
- Misses       1872     1874       +2     
- Partials      258      260       +2     

Flag	Coverage Δ
unittests	80.34% <96.22%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
mmedit/core/hooks/ema.py	96.07% <96.07%> (ø)
mmedit/core/hooks/__init__.py	100.00% <100.00%> (ø)
...ls/components/stylegan2/generator_discriminator.py	83.73% <0.00%> (-1.21%)	⬇️

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