Since you are running in Google Colab, I can reproduce the same error in Colab while downloading NVIDIA NGC .
Not sure if the ngc is compatible with Google Colab.
For workaround, please use wget to download the model instead of ngc.
For example,
!ngc registry model download-version nvidia/tao/pretrained_object_detection:resnet18
–dest $LOCAL_EXPERIMENT_DIR/pretrained_resnet18
equals to
Refer to https://docs.nvidia.com/tao/tao-toolkit/text/open_images/open_images_pt_object_detection.html → TAO Pretrained Object Detection | NVIDIA NGC