Enable FORCE_CUDA environment variable in setup.py

[c60evaporator]

What / Why

As mentioned in #111, the current setup.py ignores an environment variable FORCE_CUDA which is used for enabling GPU-based build without GPU visible environment.

Reproduce

Install the repo in non-GPU environment such as Docker build

pip install -v -e .

Then run BEVFormer eval script

./adzoo/bevformer/dist_test.sh ./adzoo/bevformer/configs/bevformer/bevformer_base_b2d.py ./ckpts/bevformer_base_b2d.pth 1

The following error occurred

ImportError: /workspace/Bench2DriveZoo/mmcv/ops/roiaware_pool3d/roiaware_pool3d_ext.cpython-38-x86_64-linux-gnu.so: undefined symbol: _Z19points_in_boxes_gpuN2at6TensorES0_S0_
E0205 05:58:29.819793 136488267859136 torch/distributed/elastic/multiprocessing/api.py:833] failed

Fix

Enable FORCE_CUDA environment variable in setup.py

Verification

Confirmed that the GPU-build is available both in GPU environments and non-GPU environments such as Docker containers.