fix the example of inference with C++ interface

[njzjz]

fix #1493

[codecov-commenter]

Codecov Report

Merging #1504 (ac30ded) into devel (17d1443) will decrease coverage by 1.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel    #1504      +/-   ##
==========================================
- Coverage   76.02%   74.56%   -1.46%     
==========================================
  Files          93       93              
  Lines        7711     7698      -13     
==========================================
- Hits         5862     5740     -122     
- Misses       1849     1958     +109     

Impacted Files	Coverage Δ
deepmd/infer/deep_tensor.py	56.09% <0.00%> (-39.03%)	⬇️
deepmd/train/run_options.py	71.42% <0.00%> (-15.37%)	⬇️
deepmd/utils/network.py	82.10% <0.00%> (-10.53%)	⬇️
deepmd/loggers/loggers.py	41.90% <0.00%> (-9.53%)	⬇️
deepmd/train/trainer.py	69.76% <0.00%> (-4.77%)	⬇️
deepmd/env.py	76.00% <0.00%> (-1.00%)	⬇️
deepmd/fit/ener.py	91.66% <0.00%> (-0.88%)	⬇️
deepmd/descriptor/se_a.py	94.15% <0.00%> (-0.67%)	⬇️
deepmd/loss/ener.py	47.57% <0.00%> (-0.23%)	⬇️
deepmd/utils/argcheck.py	90.06% <0.00%> (-0.20%)	⬇️
... and 15 more

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