initialize input virial vector to zero

[njzjz]

Fix #1123.

Thanks for debugging from @Yi-FanLi !

[codecov-commenter]

Codecov Report

Merging #1397 (ddfc628) into devel (1a51b5d) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel    #1397      +/-   ##
==========================================
+ Coverage   75.53%   75.63%   +0.09%     
==========================================
  Files          91       92       +1     
  Lines        7506     7531      +25     
==========================================
+ Hits         5670     5696      +26     
+ Misses       1836     1835       -1     

Impacted Files	Coverage Δ
deepmd/fit/ener.py	91.70% <0.00%> (ø)
deepmd/fit/fitting.py	100.00% <0.00%> (ø)
deepmd/descriptor/se_t.py	97.35% <0.00%> (+0.01%)	⬆️
deepmd/descriptor/se_r.py	91.54% <0.00%> (+0.04%)	⬆️
deepmd/fit/dipole.py	93.24% <0.00%> (+0.18%)	⬆️
deepmd/descriptor/se_a.py	94.15% <0.00%> (+0.28%)	⬆️
deepmd/fit/polar.py	49.27% <0.00%> (+0.49%)	⬆️
deepmd/common.py	81.57% <0.00%> (+1.57%)	⬆️
deepmd/descriptor/se.py	72.72% <0.00%> (+4.30%)	⬆️

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