Add flatten_sum and refactor all new transform functions

[fsxbhyy]

1. Add flatten_sum
2. Add a map to keep track of visited nodes for all transforms
3. Add documentations

[codecov-commenter]

Codecov Report

Attention: 61 lines in your changes are missing coverage. Please review.

Comparison is base (2a4419a) 61.54% compared to head (f94f13c) 61.01%.
Report is 1 commits behind head on computgraph.

Files	Patch %	Lines
src/computational_graph/transform.jl	0.00%	60 Missing ⚠️
src/computational_graph/eval.jl	88.88%	1 Missing ⚠️

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Additional details and impacted files

@@               Coverage Diff               @@
##           computgraph     #171      +/-   ##
===============================================
- Coverage        61.54%   61.01%   -0.54%     
===============================================
  Files               66       66              
  Lines             5061     5112      +51     
===============================================
+ Hits              3115     3119       +4     
- Misses            1946     1993      +47     

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