test: move loading graphs to setUpClass

[njzjz]

Loading graphs is expensive.

[codecov-commenter]

Codecov Report

Merging #1484 (d6633d8) into devel (33a1dfa) will decrease coverage by 1.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel    #1484      +/-   ##
==========================================
- Coverage   75.99%   74.87%   -1.13%     
==========================================
  Files          93       93              
  Lines        7674     7674              
==========================================
- Hits         5832     5746      -86     
- Misses       1842     1928      +86     

Impacted Files	Coverage Δ
deepmd/infer/deep_tensor.py	56.09% <0.00%> (-39.03%)	⬇️
deepmd/train/run_options.py	71.42% <0.00%> (-15.24%)	⬇️
deepmd/loggers/loggers.py	41.90% <0.00%> (-9.53%)	⬇️
deepmd/train/trainer.py	71.62% <0.00%> (-2.80%)	⬇️

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