Add cost inference for MulGradient operator
Summary: Add cost inference for MulGradient operator; also whitelist MulGradient in COMPUTE_OP_TYPES in dense_perf_estimation
Test Plan: buck run //caffe2/caffe2/python/operator_test:elementwise_ops_test
Reviewed By: CrazySherman
Differential Revision: D27614003
fbshipit-source-id: 30901e5e2b6ce7e2183c2362d1bf9f895046cf55
