Fix import tests

Context

This MR is dedicated to enhance some operators imports mentioned in issue aidge_onnx#46.
The operators that will be enhanced are the following:

• gemm: * missing alpha and beta attributes.
  * missing transA and transB attributes.
  * cannot support bias of shape [1, outChannels].
• Sum: can no longer use Add operator because the latter can no longer support more than 2 inputs.

Modified files

• MetaOperatorDefs.hpp, Gemm.cpp and pybind_MetaOperatorDefs.cpp, add Gemm metaoperator
• FC.cpp support bias of shape [1, outChannels];
• Sum.hpp, Sum.cpp and pybind_Sum.cpp, add Sum operator;

TODO

• Add Gemm metaoperator with the attributes alpha, beta, transA and transB
• Support bias of shape [1, outChannels] in FC oprator
• Add Sum operator