tests: Add running to README; relax silhouette tests to use IoU

[EricCousineau-TRI]

Just did some quick test fixes

Tested this against torch==1.7.1+cu110 on my machine, using @adambielski's fix in #86

[EricCousineau-TRI]

My full transcript on Ubuntu 18.04:

cuda-11.2-setup ()
{
    export CUDA_ROOT=/usr/local/cuda-11.2;
    export CUDA_INC_DIR=${CUDA_ROOT}/include;
    export PATH=${CUDA_ROOT}/bin:${PATH};
    export LD_LIBRARY_PATH=${CUDA_ROOT}/lib64:${LD_LIBRARY_PATH}
}

python3 -m venv ./venv
source ./venv/bin/activate
pip install -U pip wheel

pip install torch==1.7.1+cu110 -f https://download.pytorch.org/whl/torch_stable.html
cuda-11.2-setup  # Mixing CUDA 11.0 and 11.2, but meh
pip install -e .

pip install scikit-image

cd ./tests
python3 -m unittest discover -t .