refactor update_sel

[njzjz]

The current update_sel has too many complex conditionals. This PR rewrites them in the Model and Descriptor classes, which makes it easier to maintain the existing classes and implement new classes.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (f256dff) 75.46% compared to head (974f203) 75.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2901      +/-   ##
==========================================
+ Coverage   75.46%   75.49%   +0.03%     
==========================================
  Files         244      244              
  Lines       24522    24569      +47     
  Branches     1580     1580              
==========================================
+ Hits        18505    18548      +43     
- Misses       5086     5090       +4     
  Partials      931      931              

Files	Coverage Δ
deepmd/descriptor/hybrid.py	61.29% <100.00%> (+2.19%)	⬆️
deepmd/descriptor/loc_frame.py	95.41% <100.00%> (+0.10%)	⬆️
deepmd/descriptor/se.py	82.75% <100.00%> (+3.59%)	⬆️
deepmd/descriptor/se_a_ef.py	53.80% <100.00%> (+0.25%)	⬆️
deepmd/descriptor/se_atten.py	94.04% <100.00%> (+0.07%)	⬆️
deepmd/entrypoints/train.py	84.25% <100.00%> (-3.41%)	⬇️
deepmd/model/frozen.py	81.48% <100.00%> (+1.08%)	⬆️
deepmd/model/linear.py	77.38% <100.00%> (+1.43%)	⬆️
deepmd/model/multi.py	78.54% <100.00%> (+0.47%)	⬆️
deepmd/model/pairwise_dprc.py	93.75% <100.00%> (+0.25%)	⬆️
... and 2 more

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