Find detectron ops from pytorch nightly build

[darthsuogles]

Detectron ops are already installed with the latest pytorch-nightly build (from conda).

find $(dirname $(python -c 'import torch; print(torch.__file__)')) -name '*detectron*.so'
# /opt/conda/lib/python3.7/site-packages/torch/lib/libcaffe2_detectron_ops_gpu.so

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