Allow custom NodeSamplerInput to be piped into DistNeighborLoader

[kmontemayor2-sc]

Synced offline with @mkolodner-sc and we do need to provide custom logic for the nodes to sample, so let's go with this instead of #155.

We could also pass in some transform_fn to the constructor here, that gets called around 1 but that seems more kludgy than this aproach.

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