Require step methods to emit sampler stats

[michaelosthege]

As a follow-up to #6205 we should require step methods (astep in particular) to always return stats.

Doing this will allow us to simplify code in places like these:

pymc/pymc/sampling.py

Lines 976 to 983 in 9c313cb

	if step.generates_stats:
	point, stats = step.step(point)
	strace.record(point, stats)
	log_warning_stats(stats)
	diverging = i > tune and stats and stats[0].get("diverging")
	else:
	point = step.step(point)
	strace.record(point)

pymc/pymc/parallel_sampling.py

Lines 194 to 200 in 9c313cb

	def _compute_point(self):
	if self._step_method.generates_stats:
	point, stats = self._step_method.step(self._point)
	else:
	point = self._step_method.step(self._point)
	stats = None
	return point, stats