voxel_grid enhancement -> Re-indexation from 0

[tchaton]

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Hey @rusty1s,

I am using voxel_grid, and on my 8000 point cloud, it creates a 5001 indexes with the highest being around 400k.

As a result, the pool_pos used to make the aggregation create a massive tensor, which isn't great for memory reduction.

The new features will add reindex=True, return new batch and reindexed indices from 0 to N.

Best. Thomas Chaton.