Move computeOutputDims() function from GenericOp to OpTensor python binding so that more Operators can use it

Add __setattr__ and __getattr__ methods for Attributes. This recquired a change in the order of derived class declaration, see https://github.com/pybind/pybind11/issues/3804

[Fix] set FC/ConvDepthWise parameter sizes at construction by adding the number of input channels in construction parameters