convert constants (which may change) to variables

[njzjz]

Following #1592, I noticed that constants cannot be recovered when restart training.

(but it can be recovered automatically if it is a variable instead)

[codecov-commenter]

Codecov Report

Merging #1606 (602a8e2) into devel (8d4d93d) will decrease coverage by 12.27%.
The diff coverage is n/a.

❗ Current head 602a8e2 differs from pull request most recent head d507646. Consider uploading reports for the commit d507646 to get more accurate results

@@             Coverage Diff             @@
##            devel    #1606       +/-   ##
===========================================
- Coverage   76.56%   64.28%   -12.28%     
===========================================
  Files          93        5       -88     
  Lines        7817       14     -7803     
===========================================
- Hits         5985        9     -5976     
+ Misses       1832        5     -1827     

Impacted Files	Coverage Δ
deepmd/descriptor/se_a.py
deepmd/fit/ener.py
deepmd/entrypoints/__init__.py
source/op/_prod_virial_grad.py
deepmd/entrypoints/compress.py
deepmd/entrypoints/config.py
deepmd/infer/deep_dipole.py
deepmd/utils/path.py
deepmd/utils/graph.py
deepmd/utils/sess.py
... and 78 more

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