do not loop using np.append

[njzjz]

A loop using np.append is $O(N^2)$. See numpy/numpy#17090.

[codecov-commenter]

Codecov Report

Base: 75.65% // Head: 76.90% // Increases project coverage by +1.24% 🎉

Coverage data is based on head (a4cd4eb) compared to base (b474a30).
Patch coverage: 80.95% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2259      +/-   ##
==========================================
+ Coverage   75.65%   76.90%   +1.24%     
==========================================
  Files         204      182      -22     
  Lines       20259    17040    -3219     
  Branches     1438      939     -499     
==========================================
- Hits        15327    13104    -2223     
+ Misses       4025     3298     -727     
+ Partials      907      638     -269     

Impacted Files	Coverage Δ
deepmd/descriptor/se_a_ebd.py	16.21% <0.00%> (-0.12%)	⬇️
deepmd/fit/ener.py	89.29% <85.71%> (+0.06%)	⬆️
deepmd/common.py	77.98% <100.00%> (-0.21%)	⬇️
deepmd/utils/data.py	93.62% <100.00%> (-0.03%)	⬇️
deepmd/utils/data_system.py	82.50% <100.00%> (+0.13%)	⬆️
deepmd/__main__.py	0.00% <0.00%> (-100.00%)	⬇️
source/lib/src/gelu.cc	23.07% <0.00%> (-53.85%)	⬇️
source/lib/src/map_aparam.cc	0.00% <0.00%> (-50.00%)	⬇️
source/lib/src/env_mat.cc	38.32% <0.00%> (-42.52%)	⬇️
deepmd/entrypoints/convert.py	26.66% <0.00%> (-40.00%)	⬇️
... and 66 more

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