Mitigation of windows missing weak symbol support about connxr

[alrevuelta]

The whole point of this project is to have a runtime that is as simple as possible and that can be compiled in with every compiler. If we start to rely on compiler-dependant features that are not widely supported by all compilers, we are losing the purpose of the project.

My suggestion is the following. We can get rid of all extern, weak and so on, and just have normal function prototypes.

//operator__onnx__conv__11.h file

operator_status operator__onnx__conv__11__T_tensor_float(
    node_context *ctx
);
operator_status operator__onnx__conv__11__T_tensor_float16(
    node_context *ctx
);

Then we autogenerate all files + functions with some dummy implementation.

//operator__onnx__conv__11__T_tensor_float.c file
operator__onnx__conv__11__T_tensor_float(node_context *ctx)
{
  return NOT_IMPLEMENTED;
}

//operator__onnx__conv__11__T_tensor_float16.c file
operator__onnx__conv__11__T_tensor_float16(node_context *ctx)
{
  return NOT_IMPLEMENTED;
}

And then if someone wants to implement an operator, they just have to locate the function/file (operator__onnx__conv__11__T_tensor_float for example) remove the return NOT_IMPLEMENTED and implement the operator.

Maybe not as fancy as using weak attributes, but do we really need that? This would fix also #32. Why not abstracting other developers form the complexity of generating all this? Just leave a function in place ready to be implemented.