Sampling fails after init with advi+adapt_diag_grad

[fonnesbeck]

When initializing a sample run with advi+adapt_diag_grad, the init runs without error, but when sampling starts it immediately fails. It appears the tune argument is not being passed from sample:

Traceback (most recent call last):
  File "/Users/cfonnesbeck/Repositories/pymc3/pymc3/parallel_sampling.py", line 128, in run
    self._start_loop()
  File "/Users/cfonnesbeck/Repositories/pymc3/pymc3/parallel_sampling.py", line 182, in _start_loop
    point, stats = self._compute_point()
  File "/Users/cfonnesbeck/Repositories/pymc3/pymc3/parallel_sampling.py", line 209, in _compute_point
    point, stats = self._step_method.step(self._point)
  File "/Users/cfonnesbeck/Repositories/pymc3/pymc3/step_methods/arraystep.py", line 263, in step
    apoint, stats = self.astep(array)
  File "/Users/cfonnesbeck/Repositories/pymc3/pymc3/step_methods/hmc/base_hmc.py", line 170, in astep
    self.potential.update(hmc_step.end.q, hmc_step.end.q_grad, self.tune)
  File "/Users/cfonnesbeck/Repositories/pymc3/pymc3/step_methods/hmc/quadpotential.py", line 310, in update
    super().update(sample, grad)
TypeError: update() missing 1 required positional argument: 'tune'

Here is the call:

    trace = pm.sample(1000, tune=3000, return_inferencedata=True, init='advi+adapt_diag_grad')

Running current master (3.9.0) on macOS and Python 3.8.