Support accurate dataset length computation in `TFDSDataset` components in TF2.3

[AdamHillier]

From the TF 2.3 release notes:

• tf.data.experimental.cardinality is now a method on tf.data.Dataset.
• tf.data.Dataset now supports len(Dataset) when the cardinality is finite.

We should make use of these functions here:

zookeeper/zookeeper/tf/dataset.py

Lines 114 to 117 in b0b9bcf

	def num_examples(self, split) -> int:
	"""Compute the number of examples in a given split."""
	return sum(self.splits[s].num_examples for s in base_splits(split))