optimize DPTabulate._build_lower method

[njzjz]

This commit optimizes the execution time of DPTabulate._build_lower from 1s to 0.1s.

I confirm that the returned value (self.data[net]) does not change.

[codecov-commenter]

Codecov Report

Merging #1323 (22b8d06) into devel (29e59d3) will decrease coverage by 11.33%.
The diff coverage is n/a.

❗ Current head 22b8d06 differs from pull request most recent head 42f8330. Consider uploading reports for the commit 42f8330 to get more accurate results

@@             Coverage Diff             @@
##            devel    #1323       +/-   ##
===========================================
- Coverage   75.62%   64.28%   -11.34%     
===========================================
  Files          91        5       -86     
  Lines        7498       14     -7484     
===========================================
- Hits         5670        9     -5661     
+ Misses       1828        5     -1823     

Impacted Files	Coverage Δ
deepmd/utils/tabulate.py
deepmd/descriptor/se_a_ebd.py
deepmd/calculator.py
deepmd/infer/deep_pot.py
source/op/_prod_force_grad.py
deepmd/descriptor/descriptor.py
deepmd/env.py
deepmd/entrypoints/doc.py
source/op/_gelu.py
deepmd/fit/__init__.py
... and 76 more

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[denghuilu]

Why these changes matter to numpy's execution performance?

[njzjz]

Why these changes matter to numpy's execution performance?

https://numpy.org/devdocs/user/whatisnumpy.html