Fix import tests

Context

This MR is dedicated to enhance some operators imports mentioned in issue aidge_onnx#46 (moved).
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

• Test_MetaOperator.cpp add unit tests for Gemm metaoperator
• SumImpl.hpp, SumImpl.cpp, SumImpl_kernels.hpp and Test_SumImpl.cpp, add Sum operator;

TODO

• Add tests for gemm operator
• Add Sum operator