fix the version of opencv-python (#1829)
Summary:
According to the [upstream issue](https://github.com/opencv/opencv-python/issues/885), opencv-python has a compatibility problem with numpy. Since we fixed the numpy version in the requirements.txt, we also need to fix the opencv-python's verison too. I install the nightly Torch and TorchBench, there is an TorchBench installation error same with the above issue with opencv-python 4.8. But opencv-python 4.7.0.72 works well. I think we'd better fix its version until the latest opencv-python 4.9.0 released.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1829
Reviewed By: xuzhao9
Differential Revision: D48439448
Pulled By: FindHao
fbshipit-source-id: da34057de3908f30f19d9999aeab800b512a49df
