Not handled case: CUDA without CUDA_HOME set

[MiroPsota]

When installing torch from pip, from torch.utils.cpp_extension import CUDA_HOME returns None, nevetheless CUDA works just fine.

In detectron2, os.path.isdir(CUDA_HOME) in this line expects CUDA_HOME to be not None.

detectron2/detectron2/utils/collect_env.py

Line 136 in f1d0c05

msg = " - invalid!" if not os.path.isdir(CUDA_HOME) else ""

As a temporary workaround, I'm setting CUDA_HOME=/.

Environment:
As provided script is not working (it uses mentioned code), I'm just listing code to recreate env:

conda create -n detectron2 -y
conda activate detectron2

conda install python=3.8 -y

pip install \
  -f https://download.pytorch.org/whl/torch_stable.html \
  torch==1.7.0+cu101 \
  torchvision==0.8.1+cu101 \
  pycocotools \
  -f https://dl.fbaipublicfiles.com/detectron2/wheels/cu101/torch1.7/index.html \
  detectron2==0.3