Fixed CPU only install

[chcaya]

I couldn't get past the FORCE_CUDA=0 pip install -e . step for my CPU-only installation while following the installation guide. I managed to complete and test my installation with the fix proposed in this PR.

Do not forget to change the config to train.device = "cpu" in dn_detr_r50_50ep.py when running the demo test:

python demo/demo.py --config-file projects/dab_detr/configs/dab_detr_r50_50ep.py \
                    --input "./idea.jpg" \
                    --output "./demo_output.jpg" \
                    --opts train.init_checkpoint="./dab_detr_r50_50ep.pth"