do not import CUDA functions when CUDA device is not available (#198)

* Do not import CUDA functions when CUDA device is not available

* update name

* update wording

Author: yxlao

ml3d/metrics/__init__.py

@@ -1,6 +1,7 @@
 import open3d
 
-if open3d._build_config['BUILD_CUDA_MODULE']:
+if open3d.core.cuda.device_count() > 0:
+    # Open3D is built with CUDA and the machine has a CUDA device.
     from open3d.ml.contrib import iou_bev_cuda as iou_bev
     from open3d.ml.contrib import iou_3d_cuda as iou_3d
 else: