[Feature] Support conversion to y-channel when loading images

[ckkelvinchan]

Motivation

As suggested by #642, support conversion to y-channel images, since some models are trained on y-channels.

Modifications

Modify mmedit/datasets/pipelines/loading.py by adding the argument convert_to in the corresponding functions.

[codecov]

Codecov Report

Merging #643 (e22bade) into master (0ba2aae) will increase coverage by 0.07%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
+ Coverage   80.41%   80.49%   +0.07%     
==========================================
  Files         198      198              
  Lines       11169    11233      +64     
  Branches     1780     1795      +15     
==========================================
+ Hits         8982     9042      +60     
- Misses       1913     1915       +2     
- Partials      274      276       +2     

Flag	Coverage Δ
unittests	80.47% <93.33%> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
mmedit/datasets/pipelines/loading.py	96.01% <93.33%> (+0.75%)	⬆️
...ls/components/stylegan2/generator_discriminator.py	84.93% <0.00%> (-1.21%)	⬇️
mmedit/datasets/pipelines/random_degradations.py	93.89% <0.00%> (-0.01%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0ba2aae...e22bade. Read the comment docs.