Add SRFolderDatasetGT.

[Yshuo-Li]

Add SRFolderDatasetGT, only read GT image from folder.
And LQ image will be generated by DownSampling (in another PR).

[codecov]

Codecov Report

Merging #223 (de4f182) into master (79f74a1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   81.71%   81.73%   +0.02%     
==========================================
  Files         148      150       +2     
  Lines        7055     7118      +63     
  Branches     1047     1054       +7     
==========================================
+ Hits         5765     5818      +53     
- Misses       1171     1179       +8     
- Partials      119      121       +2     

Flag	Coverage Δ
unittests	81.73% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
mmedit/datasets/__init__.py	100.00% <100.00%> (ø)
mmedit/datasets/sr_folder_gt_dataset.py	100.00% <100.00%> (ø)
mmedit/models/common/generation_model_utils.py	95.19% <0.00%> (-4.81%)	⬇️
mmedit/datasets/pipelines/__init__.py	100.00% <0.00%> (ø)
mmedit/datasets/pipelines/down_sampling.py	89.13% <0.00%> (ø)

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[innerlee]

Thanks! Since our datasets have Dataset as suffix, the SRFolderDatasetGT class could be renamed to SRGTFolderDataset or SRFolderGTDataset or SRGroundTruthFolderDataset.