[MatMul] add onnx export for MatMul operator

Context

MatMul aidge operator is not supported by export_onnx() in version aidge_onnx v0.2.1

Modified files

Simply added aidge_onnx/node_export/aidge_converters/matmul.py registering export for operator MatMul.

There is not transformation done, the operator is simply exported as an onnx core MatMul op.

Detailed major modifications

Note that I do not export attributes if there are as it seems to be the standard behavior of export_onnx().

Reviewers question

• unit tests: I'm new to the project and I do not know if there are or where are the unit tests for onnx export/import. If relevant, can someone give me a ref to the tests repository?
• integration tests: otherwise maybe there are some integration tests with input onnx files, I may contribute a file containing a MatMul?
• commit message: please comment if the commit message is not conformant, I will update

TODO

• Not necessary: Unit test if relevant
• Not necessary: Integration test if relevant
• OK: Commit message