Add missing __len__ impl to Mixture selector

[AndrejOrsula]

First off, thanks for the great work!

Issue Description

Sampling with the Mixture selector results in an error due to missing __len__() implementation.

Traceback (most recent call last):
  ...
  File "dreamerv3/embodied/core/replay.py", line 123, in <lambda>
    limiters.wait(lambda: len(self.sampler), message)
TypeError: object of type 'Mixture' has no len()

dreamerv3/embodied/core/replay.py

Line 123 in 55c8c67

limiters.wait(lambda: len(self.sampler), message)

PR Considerations

The implemented __len__() simply dispatches the call to the first selector. Reasoning:

• At least one selector is guaranteed to exist after Mixture.__init__():

dreamerv3/embodied/core/selectors.py

Lines 202 to 210 in 55c8c67

	def __init__(self, selectors, fractions, seed=0):
	assert set(selectors.keys()) == set(fractions.keys())
	assert sum(fractions.values()) == 1, fractions
	for key, frac in list(fractions.items()):
	if not frac:
	selectors.pop(key)
	fractions.pop(key)
	keys = sorted(selectors.keys())
	self.selectors = [selectors[key] for key in keys]

• The output of len() for each selector is identical because they have the same items:

dreamerv3/embodied/core/selectors.py

Lines 217 to 223 in 55c8c67

	def __setitem__(self, key, stepids):
	for selector in self.selectors:
	selector[key] = stepids
	def __delitem__(self, key):
	for selector in self.selectors:
	del selector[key]