[Bug Fix] Update dist_neighbor_sampler to handle heterogeneous case where node labels is None

[mkolodner-sc]

This PR aims to fix dist_neighbor_sampler which fails in the heterogeneous case when self.dist_node_labels=None. There is an assert statement which enforces this field to be a dictionary, but the subsequent line implies that we should be allowing for this to also be None:

if self.dist_node_labels is not None and all(isinstance(value, Feature) for value in self.dist_node_labels.values()):

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