ReduceMean change its axes attribute during forwardDims

During forwardDims, ReduceMean update axes to make them positive.

I think this is a bad behavior as forwardDims should only forward the dimensions.

Also, this is confusing has when I import the ONNX the axes are [-2, -1] and then changed to [1, 2]. While it is still technically equivalent I don't think we need to make the axes positives during forwardDims.