Support start_index in GenerateSegmentIndices

[ckkelvinchan]

Motivation

As mentioned in #335, the existing video_restoration_demo does not support sequences whose start frame index is not 0. This may cause problems in custom video sequences.

Modification

This PR changes the behavior of the function GenerateSegmentIndices so that the image names can now start from number other than 0. Changes are:

1. Instead of assuming the start frame index is 0, we add the value start_idx to the extracted indices.
2. video_restoration_demo and video_restoration_inference are modified.

Note: The image names must be consecutive. That means, a sequence with images 0000.png, 0004.png, 0020.png is not supported.

[codecov]

Codecov Report

Merging #338 (8700796) into master (6238c69) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #338   +/-   ##
=======================================
  Coverage   80.03%   80.04%           
=======================================
  Files         178      178           
  Lines        9332     9334    +2     
  Branches     1357     1358    +1     
=======================================
+ Hits         7469     7471    +2     
  Misses       1674     1674           
  Partials      189      189           

Flag	Coverage Δ
unittests	80.03% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
mmedit/apis/restoration_video_inference.py	21.42% <100.00%> (ø)
mmedit/datasets/pipelines/augmentation.py	97.06% <100.00%> (+0.01%)	⬆️
...ls/components/stylegan2/generator_discriminator.py	84.93% <0.00%> (ø)

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