[fx2trt] Factor out add_matrix_multiply_layer
Summary: Factor out the function so that it can be reused in future diffs
Test Plan: buck run mode/opt caffe2/torch/fb/fx2trt:test_matmul
Reviewed By: 842974287
Differential Revision: D30322823
fbshipit-source-id: 069b945de2c744cdbcca1618b62827692dfb4174
