Copy and paste the following snippet in a colab cell. A slightly improved version of the parent gist, getting you the correct public address.

SSH to Colaboratory VM.txt

#Generate root password
import random, string
password = ''.join(random.choice(string.ascii_letters + string.digits) for i in range(20))

#Download ngrok
! wget -q -c -nc https://bin.equinox.io/c/4VmDzA7iaHb/ngrok-stable-linux-amd64.zip
! unzip -qq -n ngrok-stable-linux-amd64.zip
#Setup sshd
! apt-get install -qq -o=Dpkg::Use-Pty=0 openssh-server pwgen > /dev/null
#Set root password
! echo root:$password | chpasswd
! mkdir -p /var/run/sshd
! echo "PermitRootLogin yes" >> /etc/ssh/sshd_config
! echo "PasswordAuthentication yes" >> /etc/ssh/sshd_config
! echo "LD_LIBRARY_PATH=/usr/lib64-nvidia" >> /root/.bashrc
! echo "export LD_LIBRARY_PATH" >> /root/.bashrc

#Run sshd
get_ipython().system_raw('/usr/sbin/sshd -D &')

#Ask token
print("Copy authtoken from https://dashboard.ngrok.com/auth")
import getpass
authtoken = getpass.getpass()

#Create tunnel
get_ipython().system_raw('./ngrok authtoken $authtoken && ./ngrok tcp 22 &')
#Get public address
! curl -s http://localhost:4040/api/tunnels | python3 -c \
    "import sys, json; print('Public address: root@', json.load(sys.stdin)['tunnels'][0]['public_url'][6:], sep='')"
#Print root password
print("Root password: {}".format(password))