Add MultipleGT datasets

[ckkelvinchan]

This PR adds three datasets that are used for training recurrent networks for video super-resolution. These networks require multiple GTs during training. The three datasets are

1. SRREDSMultipleGTDataset: It is used to train and test on REDS dataset.
2. SRVimeo90KMultipleGTDataset: It is used to train on Vimeo-90K dataset.
3. SRTestMultipleGTDataset: It is used to test on various datasets such as Vid4 and UDM10.

[codecov]

Codecov Report

Merging #238 (712848b) into master (3a04b6e) will increase coverage by 0.16%.
The diff coverage is 100.00%.

❗ Current head 712848b differs from pull request most recent head 91480a2. Consider uploading reports for the commit 91480a2 to get more accurate results

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   81.41%   81.57%   +0.16%     
==========================================
  Files         153      156       +3     
  Lines        7413     7480      +67     
  Branches     1088     1102      +14     
==========================================
+ Hits         6035     6102      +67     
  Misses       1247     1247              
  Partials      131      131              

Flag	Coverage Δ
unittests	81.57% <100.00%> (+0.16%)	⬆️

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Impacted Files	Coverage Δ
mmedit/datasets/__init__.py	100.00% <100.00%> (ø)
mmedit/datasets/sr_reds_multiple_gt_dataset.py	100.00% <100.00%> (ø)
mmedit/datasets/sr_test_multiple_gt_dataset.py	100.00% <100.00%> (ø)
mmedit/datasets/sr_vimeo90k_multiple_gt_dataset.py	100.00% <100.00%> (ø)

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