Fix: astar bugs

[heatingma]

1. fix bugs when use astar with cuda
2. Resolve the error occurred when the number of nodes in graph1 inputted by astar is greater than the number of nodes in graph2

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c537694) 92.78% compared to head (2404be0) 92.74%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
pygmtools/pytorch_backend.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
- Coverage   92.78%   92.74%   -0.05%     
==========================================
  Files          20       20              
  Lines        7667     7679      +12     
==========================================
+ Hits         7114     7122       +8     
- Misses        553      557       +4     

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