add examples to docstings

[njzjz]

Directly copied from doc/inference/python.md.

[codecov-commenter]

Codecov Report

Merging #1010 (fbdd401) into devel (472d7c4) will decrease coverage by 10.31%.
The diff coverage is n/a.

❗ Current head fbdd401 differs from pull request most recent head 9165b3f. Consider uploading reports for the commit 9165b3f to get more accurate results

@@             Coverage Diff             @@
##            devel    #1010       +/-   ##
===========================================
- Coverage   74.59%   64.28%   -10.32%     
===========================================
  Files          86        5       -81     
  Lines        6921       14     -6907     
===========================================
- Hits         5163        9     -5154     
+ Misses       1758        5     -1753     

Impacted Files	Coverage Δ
source/op/_tabulate_grad.py
deepmd/entrypoints/config.py
deepmd/fit/ener.py
deepmd/train/run_options.py
deepmd/__main__.py
source/op/_prod_force_se_r_grad.py
deepmd/loss/__init__.py
deepmd/entrypoints/test.py
source/op/_prod_force_grad.py
deepmd/fit/dipole.py
... and 69 more

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