Ratio for num_neighbors in NeighborLoader / LinkNeighborLoader

[bryceForrest]

🚀 The feature, motivation and pitch

I think it might be cool and useful to have the option to pass a float in (0, 1] for num_neighbors. If nodes have have highly varied degrees, a raw count of, say, 5 neighbors may be a substantial amount of information node u and significantly less information for node v. It may be more consistent to sample 0.5 of u's neighbors and 0.5 of v's neighbors.

Alternatives

I actually have a very hard time wrapping my brain around writing my own PyG loaders. Idk if that's because the code is scattered across files or what, but I'm not sure where I would begin to write my own.

Additional context

No response