enable model compression for dipole and polar type fitting net

[denghuilu]

main changes:

• add init_variables for dipole and polar fitting net
• change the parameter of the init_variables function to model_file in the fitting net to match the form of init_variables in descriptors.

This PR should fix issue #1226 .

[codecov-commenter]

Codecov Report

Merging #1228 (db5c821) into devel (1cf3e50) will decrease coverage by 0.01%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##            devel    #1228      +/-   ##
==========================================
- Coverage   76.03%   76.02%   -0.02%     
==========================================
  Files          91       91              
  Lines        7358     7367       +9     
==========================================
+ Hits         5595     5601       +6     
- Misses       1763     1766       +3     

Impacted Files	Coverage Δ
deepmd/descriptor/se.py	68.42% <ø> (ø)
deepmd/train/trainer.py	73.89% <50.00%> (ø)
deepmd/fit/polar.py	48.74% <70.00%> (+0.55%)	⬆️
deepmd/fit/dipole.py	95.65% <85.71%> (-1.28%)	⬇️
deepmd/fit/ener.py	94.60% <100.00%> (-0.03%)	⬇️

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