[RFE] Add disjoint sampling support to NeighborLoader and LinkNeighborLoader

[paulhendricks]

Problem

Disjoint sampling is explicitly rejected. NeighborLoader and LinkNeighborLoader raise a ValueError if disjoint=True.

This blocks users relying on PyG's disjoint mini-batches.

Expected behavior

Support disjoint sampling that produces disjoint subgraphs in line with PyG behavior.