Add Tensor Graph

[fsxbhyy]

Added TensorGraph as a new type of graph that supports tensor operation
Compared to Graph (for scalar):

1. Add a shape field in TensorGraph to track dimension of tensor.
2. Removed Prod(), Power() operators and related functions for tensorgraph multiplication.
3. Added EinSum operator and function following numpy interface. Currently is restricted to be binary operation.

[codecov-commenter]

Codecov Report

Attention: 66 lines in your changes are missing coverage. Please review.

Comparison is base (e21e062) 56.17% compared to head (8cb7e10) 56.20%.

Files	Patch %	Lines
src/computational_graph/tensorgraph.jl	56.86%	66 Missing ⚠️

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Additional details and impacted files

@@               Coverage Diff               @@
##           computgraph     #175      +/-   ##
===============================================
+ Coverage        56.17%   56.20%   +0.02%     
===============================================
  Files               51       52       +1     
  Lines             4539     4692     +153     
===============================================
+ Hits              2550     2637      +87     
- Misses            1989     2055      +66     

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