Support loss normalization

[YWolfeee]

This commit add support to loss normalization, and change the input args in "loss"

Haotian Ye,
Apr, 20, 2021

[codecov-commenter]

Codecov Report

Merging #538 (c4a9a33) into devel (ab49c43) will decrease coverage by 0.05%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##            devel     #538      +/-   ##
==========================================
- Coverage   62.15%   62.09%   -0.06%     
==========================================
  Files          74       74              
  Lines        6220     6229       +9     
==========================================
+ Hits         3866     3868       +2     
- Misses       2354     2361       +7     

Impacted Files	Coverage Δ
deepmd/utils/argcheck.py	16.54% <10.00%> (-0.13%)	⬇️
deepmd/loss/tensor.py	32.53% <100.00%> (+0.53%)	⬆️

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