[Refactor] Refactor all utils in models

[MeowZheng]

Modification

1. remove utils in encoders and decoders to models/utils
2. add unit tests of utils

[codecov]

Codecov Report

Merging #50 (9826727) into master (b34f080) will increase coverage by 1.56%.
The diff coverage is 72.97%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   65.50%   67.06%   +1.56%     
==========================================
  Files          82       83       +1     
  Lines        4711     4716       +5     
  Branches      677      677              
==========================================
+ Hits         3086     3163      +77     
+ Misses       1528     1450      -78     
- Partials       97      103       +6     

Flag	Coverage Δ
unittests	67.06% <72.97%> (+1.56%)	⬆️

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

Impacted Files	Coverage Δ
mmflow/models/utils/basic_encoder.py	94.23% <ø> (ø)
mmflow/models/utils/correlation_block.py	60.00% <60.00%> (ø)
mmflow/models/decoders/irrpwc_decoder.py	21.28% <100.00%> (ø)
mmflow/models/decoders/liteflownet_decoder.py	43.96% <100.00%> (ø)
mmflow/models/decoders/maskflownet_decoder.py	34.51% <100.00%> (ø)
mmflow/models/decoders/pwcnet_decoder.py	71.55% <100.00%> (ø)
mmflow/models/encoders/flownet_encoder.py	65.78% <100.00%> (-0.88%)	⬇️
mmflow/models/encoders/liteflownet_encoder.py	100.00% <100.00%> (ø)
mmflow/models/encoders/pwcnet_encoder.py	100.00% <100.00%> (ø)
mmflow/models/utils/__init__.py	100.00% <100.00%> (ø)
... and 6 more

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