Do not link torch_python with nccl (#37040)
Summary:
If NCCL is used, just allow `torch_python` to access enums defined in header file
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37040
Test Plan: `cmake ../pytorch -DPYTHON_EXECUTABLE=/usr/bin/python3.7 -DCMAKE_BUILD_TYPE=RELWITHDEBINFO -DUSE_CUDA=YES -DBUILD_TEST=YES -DUSE_NCCL=YES -DUSE_DISTRIBUTED=NO -DCMAKE_CXX_COMPILER=/usr/bin/cuda-g++ -DCMAKE_C_COMPILER=/usr/bin/cuda-gcc -DUSE_MKLDNN=ON -G Ninja` + `ninja torch_python`
Differential Revision: D21171573
Pulled By: malfet
fbshipit-source-id: e5eba0f610da3b0fcd17342ad46458dc7b0d251b
