Model compression checkpoint

[denghuilu]

This PR change the default model compression checkpoint path from '.' to 'model-compression'.

In the original implementation, the model compression process will overwrite the checkpoint in the current directory, which may cause damage to the checkpoint of the original model.

[codecov-commenter]

Codecov Report

Merging #1076 (4bb2a10) into devel (75160c7) will decrease coverage by 10.90%.
The diff coverage is n/a.

❗ Current head 4bb2a10 differs from pull request most recent head 701fbff. Consider uploading reports for the commit 701fbff to get more accurate results

@@             Coverage Diff             @@
##            devel    #1076       +/-   ##
===========================================
- Coverage   75.19%   64.28%   -10.91%     
===========================================
  Files          82        5       -77     
  Lines        6945       14     -6931     
===========================================
- Hits         5222        9     -5213     
+ Misses       1723        5     -1718     

Impacted Files	Coverage Δ
deepmd/descriptor/hybrid.py
deepmd/model/ener.py
deepmd/__main__.py
deepmd/descriptor/__init__.py
source/op/_prod_force_se_r_grad.py
deepmd/infer/__init__.py
deepmd/utils/data_system.py
deepmd/utils/compat.py
deepmd/calculator.py
deepmd/fit/polar.py
... and 75 more

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