[nnc] Add benchmarks
Summary: Adding a set of benchmarks for key operators
Test Plan:
buck build mode/opt -c 'fbcode.caffe2_gpu_type=none' caffe2/benchmarks/cpp/tensorexpr:tensorexpr_bench
OMP_NUM_THREADS=1 MKL_NUM_THREADS=1 numactl -C 3 ./buck-out/gen/caffe2/benchmarks/cpp/tensorexpr/tensorexpr_bench
Reviewed By: ZolotukhinM
Differential Revision: D25981260
fbshipit-source-id: 17681fc1527f43ccf9bcc80704415653a627b396
