Support MatMul in c2_pt_converter
Summary: Added the MatMul operator for caffe2
Test Plan: buck test //caffe2/torch/fb/model_transform/c2_convert:c2_pt_converter_test
Reviewed By: bugra
Differential Revision: D24920937
fbshipit-source-id: 7ba09ba0439cb9bd15d6a41fd8ff1a86d8d11437
