ndarray: change internal api of mul/div to help autograd

[iblislin]

address #274 (comment)

[codecov-io]

Codecov Report

Merging #366 into master will decrease coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   70.28%   70.11%   -0.18%     
==========================================
  Files          24       24              
  Lines        1942     1944       +2     
==========================================
- Hits         1365     1363       -2     
- Misses        577      581       +4

Impacted Files	Coverage Δ
src/ndarray.jl	85.15% <100%> (-1.05%)	⬇️

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