Allow to filter out inputs in caffe2_net_to_onnx_model

[dzhulgakov]

Caffe2 nets usually have stubs for the inputs in a form of ConstantFill in the init_net. It causes caffe2_net_to_onnx_model to complain as it doesn't know how to handle this op.

Instead we should just allow user to specify the list of model inputs and filter them out from the init_net. Also, we can extract ValueInfo from the shape to make things nicer.