[Feature] Add video interpolation demo

[Yshuo-Li]

motivation

1. Add demo script of video frame interpolation.
2. Modify the demo part of basic_interpolator.py according to demo script.

modification

1. Add video frame interpolation demo.
2. Modify the demo part of basic_interpolator.py

[codecov]

Codecov Report

Merging #717 (704a50b) into master (933220a) will increase coverage by 0.14%.
The diff coverage is 92.53%.

@@            Coverage Diff             @@
##           master     #717      +/-   ##
==========================================
+ Coverage   81.80%   81.94%   +0.14%     
==========================================
  Files         211      212       +1     
  Lines       11896    11955      +59     
  Branches     1912     1922      +10     
==========================================
+ Hits         9731     9796      +65     
+ Misses       1869     1861       -8     
- Partials      296      298       +2     

Flag	Coverage Δ
unittests	81.91% <92.53%> (+0.13%)	⬆️

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

Impacted Files	Coverage Δ
mmedit/apis/video_interpolation_inference.py	91.22% <91.22%> (ø)
mmedit/apis/__init__.py	100.00% <100.00%> (ø)
...t/models/video_interpolators/basic_interpolator.py	97.32% <100.00%> (+0.02%)	⬆️
...ls/components/stylegan2/generator_discriminator.py	86.14% <0.00%> (+1.20%)	⬆️
mmedit/models/video_interpolators/cain.py	100.00% <0.00%> (+4.00%)	⬆️
mmedit/apis/matting_inference.py	39.02% <0.00%> (+19.51%)	⬆️

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