Add graph documentations

[qianqianzhu]

Description

This PR mainly gets APIs related to graph documented.

List of changes:

• Add graph documentation (overview) graph.md
• Add API documentations for src/graph/expression_graph.h
• Add API documentations for src/graph/node_initializers.h
• Small updates on code documentation in:
  • src/graph/expression_graph.cpp
  • src/graph/node_initializers.cpp
  • src/tensors/tensor.h
  • src/graph/node.h
  • src/graph/node.cpp
  • src/common/definitions.h
  • src/common/shape.h
  • src/common/types.h
  • src/graph/node_operators.h

Added dependencies: none

How to test

Build and test successfully in gna.

Checklist

• I have tested the code manually
• I have run regression tests
• I have read and followed CONTRIBUTING.md
• I have updated CHANGELOG.md

[snukky]

Thanks a lot for this tremendous work towards having a well documented Marian one day! :)

I added a first portion of comments. General comments to the tutorial/graph documentation:

• I think we could focus on the public API and more on how to use the graph than on details how it's implemented.
• I'm missing the description on how to use DEBUG.

For the doxygen documentation, I would like to make it as minimalistic as possible, to increase the chance that people will be using it when creating new code. We should also consider creating a very simple documentation explaining basic doxygen commands, so that developers can refer to when documenting their code.

[qianqianzhu]

Just marked all Roman's comments as resolved.

[snukky]

Thanks! Doxygen comments looks much better IMO. Have you tested how does the automatic @brief work?

[graemenail]

A nice document on expression graphs with useful examples. My comments are mostly typos, and just clarifications

[qianqianzhu]

A nice document on expression graphs with useful examples. My comments are mostly typos, and just clarifications

Many thanks for your detailed comments! I just fixed them.