[Feature] Support denoising demo

[LeoXing1996]

• Unit test will be added after configs are merged.

[codecov]

Codecov Report

Merging #202 (83a2e60) into master (5cea51f) will increase coverage by 0.10%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   76.62%   76.73%   +0.10%     
==========================================
  Files         130      130              
  Lines        9396     9421      +25     
  Branches     1891     1899       +8     
==========================================
+ Hits         7200     7229      +29     
+ Misses       1729     1727       -2     
+ Partials      467      465       -2     

Flag	Coverage Δ
unittests	76.73% <92.00%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
mmgen/apis/__init__.py	100.00% <ø> (ø)
mmgen/apis/inference.py	57.50% <92.00%> (+9.07%)	⬆️
...chitectures/stylegan/generator_discriminator_v1.py	88.70% <0.00%> (-1.13%)	⬇️
...chitectures/stylegan/generator_discriminator_v2.py	87.77% <0.00%> (+1.11%)	⬆️
mmgen/models/diffusions/base_diffusion.py	87.46% <0.00%> (+1.63%)	⬆️

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