make nms import optional

nndet/core/boxes/nms.py

@@ -15,11 +15,16 @@ limitations under the License.
 """
 
 import torch
+from loguru import logger
 from torch import Tensor
 from torch.cuda.amp import autocast
 from torchvision.ops.boxes import nms as nms_2d
 
-from nndet._C import nms as nms_gpu
+try:
+    from nndet._C import nms as nms_gpu
+except ImportError:
+    logger.warning("nnDetection was not build with GPU support!")
+    nms_gpu = None
 from nndet.core.boxes.ops import box_iou