code: def _step_func(self, func, t0, dt, t1, y0):
      f0 = func(
          t0, y0, perturb=Perturb.NEXT if self.perturb else Perturb.NONE
      )
  
      butcher_tableu = [
          [0.0, 0.0, 0.0, 0.0],
          [1 / 3, 1 / 3, 0.0, 0.0],
          [2 / 3, 0.0, 2 / 3, 0.0],
          [0.0, 1 / 4, 0.0, 3 / 4],
      ]
  
      return (
          rk3_step_func(
              func,
              t0,
              dt,
              t1,
              y0,
              butcher_tableu=butcher_tableu,
              f0=f0,
              perturb=self.perturb,
          ),
          f0,
      )
  
file_path: torchdiffeq/torchdiffeq/_impl/fixed_grid.py
imports: from .solvers import FixedGridODESolver
  from .rk_common import rk4_alt_step_func, rk3_step_func
  from .misc import Perturb
label_desc: Use image node_id 1 for calling the Heun3 obj's underlying member method code with example usage: obj._step_func(func, t0, dt, t1, y0) and returns: rk3_step_func,  f0