2022-02-27 Xop, easy to create onnx graph

onnx package has a very verbose API to create ONNX graph. Could you imagine a user to directly write the syntax tree of a program instead of some python code? Creating a ONNX graph is very similar to that task except ONNX language is more simple than python.

We could start writing a compiler for ONNX language but it should be defined first. Another option consists in using an existing API, such as numpy API (see Create custom ONNX graphs with AST). But it is not always easy to keep the same simplicity when numpy is not strongly typed and ONNX is. Another direction is to implement ONNX Operators as function. Adding an operator into a graph becomes similar to a function call. This API is introduced in Xop API.