nn: support multi-categories classification

[zhenghaoz]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 89.03226% with 17 lines in your changes missing coverage. Please review.

Project coverage is 67.62%. Comparing base (5c8f681) to head (3e42711).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
common/nn/functions.go	45.45%	3 Missing and 3 partials ⚠️
common/nn/tensor.go	94.04%	5 Missing ⚠️
common/nn/op.go	91.11%	4 Missing ⚠️
common/nn/layers.go	88.88%	1 Missing ⚠️
model/click/deepfm_v2.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #915      +/-   ##
==========================================
+ Coverage   65.99%   67.62%   +1.63%     
==========================================
  Files          86       92       +6     
  Lines       18147    19556    +1409     
==========================================
+ Hits        11976    13225    +1249     
- Misses       5138     5231      +93     
- Partials     1033     1100      +67     

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