Not sure what is your intentions, but in general we discouraged user to use hmc directly, as it need extra care of choosing hyper parameters; and since internally we are using slightly different parameterization, finding the right parameters and its mapping to the input arg is not trivial - it just ends up really frustrating.
So I suggest you to use pm.sample with the default option, and tuning of hyper parameters like mass matrix and step size will be tuned to optimal automatically.
If for some reason you really need to use HMC, you can run the default sampler, and get the hyper parameters from the trace.