fix gamgm backward func & doc (jittor)

[codecov[bot]]

Codecov Report

Merging #37 (beb464a) into main (c7951fc) will increase coverage by 0.02%. The diff coverage is 23.07%.

:exclamation: Current head beb464a differs from pull request most recent head 60d39f1. Consider uploading reports for the commit 60d39f1 to get more accurate results

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
+ Coverage   69.19%   69.21%   +0.02%     
==========================================
  Files          23       23              
  Lines        6800     6806       +6     
==========================================
+ Hits         4705     4711       +6     
  Misses       2095     2095              

Impacted Files	Coverage Δ
pygmtools/multi_graph_solvers.py	80.00% <ø> (ø)
pygmtools/jittor_backend.py	84.11% <23.07%> (+0.01%)	:arrow_up:
pygmtools/numpy_backend.py	89.37% <0.00%> (+0.01%)	:arrow_up:
pygmtools/pytorch_backend.py	89.41% <0.00%> (+0.01%)	:arrow_up:
pygmtools/paddle_backend.py	86.22% <0.00%> (+0.02%)	:arrow_up:
pygmtools/tensorflow_backend.py	81.69% <0.00%> (+0.05%)	:arrow_up:
pygmtools/utils.py	72.75% <0.00%> (+0.07%)	:arrow_up:

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