[Enhance] Optimize IO for flow_warp

[NK-CS-ZZL]

This version of flow_warp is faster than original one and the consistency of results between two implementations has been checked.

[codecov]

Codecov Report

Merging #881 (77d56cc) into master (52f6728) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #881      +/-   ##
==========================================
+ Coverage   83.14%   83.15%   +0.01%     
==========================================
  Files         221      221              
  Lines       12487    12488       +1     
  Branches     2026     2026              
==========================================
+ Hits        10382    10385       +3     
+ Misses       1787     1786       -1     
+ Partials      318      317       -1     

Flag	Coverage Δ
unittests	83.11% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmedit/models/common/flow_warp.py	100.00% <100.00%> (ø)
...ls/components/stylegan2/generator_discriminator.py	86.14% <0.00%> (+1.20%)	⬆️

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[wangruohui]

Fix #878

[OpenMMLab-Assistant-004]

Hi @NK-CS-ZZL ！First of all, we want to express our gratitude for your significant PR in the mmediting project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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