code: def _single_step_verlet(
      z, r, potential_fn, kinetic_grad, step_size, z_grads=None
  ):
      r"""
      Single step velocity verlet that modifies the `z`, `r` dicts in place.
      """
  
      z_grads = (
          potential_grad(potential_fn, z)[0]
          if z_grads is None
          else z_grads
      )
  
      for site_name in r:
          r[site_name] = r[site_name] + 0.5 * step_size * (
              -z_grads[site_name]
          )  # r(n+1/2)
  
      r_grads = kinetic_grad(r)
      for site_name in z:
          z[site_name] = (
              z[site_name] + step_size * r_grads[site_name]
          )  # z(n+1)
  
      z_grads, potential_energy = potential_grad(potential_fn, z)
      for site_name in r:
          r[site_name] = r[site_name] + 0.5 * step_size * (
              -z_grads[site_name]
          )  # r(n+1)
  
      return z, r, z_grads, potential_energy
  
file_path: pyro/pyro/ops/integrator.py
imports: import warnings
  from typing import Callable, Dict
  from torch.autograd import grad
label_desc: Use image node_id 2 for calling a global function with example usage: _single_step_verlet(z, r, potential_fn, kinetic_grad, step_size, z_grads) and returns: z,  r,  z_grads,  potential_energy