[Feature] Add script to crop REDS images into sub-images for faster IO

[ckkelvinchan]

Motivation

Add a script to crop REDS images into sub-images, for faster IO. Similar to the operation for DIV2K.

Modification

1. Add tools/data/super-resolution/reds/crop_sub_images.py
2. Modify README

[codecov]

Codecov Report

Merging #669 (5404598) into master (37547f6) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
+ Coverage   80.61%   80.68%   +0.06%     
==========================================
  Files         199      199              
  Lines       11351    11389      +38     
  Branches     1814     1824      +10     
==========================================
+ Hits         9151     9189      +38     
  Misses       1916     1916              
  Partials      284      284              

Flag	Coverage Δ
unittests	80.65% <ø> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
mmedit/datasets/pipelines/random_degradations.py	94.13% <0.00%> (-0.55%)	⬇️
mmedit/datasets/pipelines/__init__.py	100.00% <0.00%> (ø)
mmedit/datasets/pipelines/crop.py	97.35% <0.00%> (+0.31%)	⬆️
mmedit/datasets/pipelines/blur_kernels.py	84.55% <0.00%> (+1.47%)	⬆️

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