[OpBench] change relu entry point after D24747035
Summary: D24747035 (https://github.com/pytorch/pytorch/commit/1478e5ec2aa42b2a9742257642c7c1d3203d7309) removes the entry point of `nnq.functional.relu`. Adjust op benchmark to `torch.nn.ReLU` accordingly.
Test Plan: buck run caffe2/benchmarks/operator_benchmark/pt:qactivation_test -- --use_jit --iterations 1 --warmup_iterations 1
Reviewed By: mingzhe09088
Differential Revision: D24961625
fbshipit-source-id: 5ed0ec7fa6d8cfefc8e7fc8324cf9a2a3e59de90
