[Feature] Add SRFolderVideoDataset

[ckkelvinchan]

This is a follow-up to #355. This PR adds support to training on arbitrary datasets for sliding-window methods, such as EDVR and TDAN.

[codecov]

Codecov Report

Merging #370 (2a76ab3) into master (2c21fa8) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 2a76ab3 differs from pull request most recent head 5816f13. Consider uploading reports for the commit 5816f13 to get more accurate results

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
- Coverage   80.30%   80.24%   -0.07%     
==========================================
  Files         184      185       +1     
  Lines        9831     9811      -20     
  Branches     1427     1441      +14     
==========================================
- Hits         7895     7873      -22     
- Misses       1743     1744       +1     
- Partials      193      194       +1     

Flag	Coverage Δ
unittests	80.24% <100.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
mmedit/datasets/sr_folder_multiple_gt_dataset.py	100.00% <ø> (ø)
mmedit/datasets/__init__.py	100.00% <100.00%> (ø)
mmedit/datasets/pipelines/augmentation.py	96.97% <100.00%> (-0.09%)	⬇️
mmedit/datasets/sr_folder_video_dataset.py	100.00% <100.00%> (ø)
...ackbones/encoder_decoders/pconv_encoder_decoder.py	43.47% <0.00%> (-2.36%)	⬇️
mmedit/models/base.py	82.75% <0.00%> (-2.09%)	⬇️
...ones/encoder_decoders/two_stage_encoder_decoder.py	53.84% <0.00%> (-1.16%)	⬇️
...dels/backbones/encoder_decoders/encoders/resnet.py	66.01% <0.00%> (-0.97%)	⬇️
...s/backbones/encoder_decoders/gl_encoder_decoder.py	81.48% <0.00%> (-0.67%)	⬇️
mmedit/datasets/pipelines/normalization.py	88.88% <0.00%> (-0.59%)	⬇️
... and 24 more

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[innerlee]

Need rebase