Multitask is available in 3.0 docker instead of 3.0-dp docker.
You need to update the tlt version.
pip3 install --upgrade nvidia-tlt
Please download jupyter notebook from https://docs.nvidia.com/tlt/tlt-user-guide/text/tlt_quick_start_guide.html#use-the-examples
wget --content-disposition https://api.ngc.nvidia.com/v2/resources/nvidia/tlt_cv_samples/versions/v1.1.0/zip -O tlt_cv_samples_v1.1.0.zip
unzip -u tlt_cv_samples_v1.1.0.zip -d ./tlt_cv_samples_v1.1.0 && rm -rf tlt_cv_samples_v1.1.0.zip && cd ./tlt_cv_samples_v1.1.0
More info is in https://docs.nvidia.com/tlt/tlt-user-guide/text/multitask_image_classification.html