[Features] Add GMA algorithm

[MeowZheng]

Modification

1. Add gma_decoder.py
2. Add gma.py config file
3. Revise raft_decoder.py
4. Revise mmflow/models/init.py
5. mmflow/models/decoders/init.py

[codecov]

Codecov Report

Merging #32 (57f16f8) into master (3d064bf) will increase coverage by 0.76%.
The diff coverage is 96.11%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   66.92%   67.68%   +0.76%     
==========================================
  Files          83       84       +1     
  Lines        4726     4828     +102     
  Branches      678      689      +11     
==========================================
+ Hits         3163     3268     +105     
+ Misses       1460     1455       -5     
- Partials      103      105       +2     

Flag	Coverage Δ
unittests	67.68% <96.11%> (+0.76%)	⬆️

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

Impacted Files	Coverage Δ
mmflow/models/__init__.py	100.00% <ø> (ø)
mmflow/models/decoders/gma_decoder.py	95.91% <95.91%> (ø)
mmflow/models/decoders/__init__.py	100.00% <100.00%> (ø)
mmflow/models/decoders/raft_decoder.py	93.71% <100.00%> (+0.10%)	⬆️
mmflow/datasets/pipelines/transforms.py	78.66% <0.00%> (+1.16%)	⬆️

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