feat: convert model files between backends

[njzjz]

dp convert-backend model.pb model.pth
dp convert-backend model.pb model.dp

[codecov]

Codecov Report

Attention: Patch coverage is 80.27211% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 75.33%. Comparing base (cf21b7a) to head (df642da).

Files	Patch %	Lines
deepmd/pt/utils/serialization.py	64.51%	11 Missing ⚠️
deepmd/entrypoints/convert_backend.py	0.00%	8 Missing ⚠️
deepmd/tf/utils/serialization.py	89.58%	5 Missing ⚠️
deepmd/entrypoints/main.py	0.00%	3 Missing ⚠️
deepmd/backend/backend.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3323      +/-   ##
==========================================
+ Coverage   75.28%   75.33%   +0.04%     
==========================================
  Files         403      406       +3     
  Lines       33888    34025     +137     
  Branches     1604     1604              
==========================================
+ Hits        25514    25632     +118     
- Misses       7513     7532      +19     
  Partials      861      861              

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[wanghan-iapcm]

model_param may be confused with the trainable model parameters. I would suggest using more self-explanatory names like model_def_script.