text: Title: Zero HMC acceptance rate for standard normal
  Body: Step size tuning in HMC fails on a standard Gaussian. Acceptance rate is 0, regardless of the step size. The problem goes away with NUTS. 
  The problem is shown in this notebook: 
  https://colab.research.google.com/drive/1QBrROnhUEAOcP3Q9-pT8AfwFjZdu9y1y?usp=sharing