Add n-dimensional grid graph generators

[Sriram-098]

Adds nd_grid_graph() and directed_nd_grid_graph() for creating n-dimensional grids.

Fixes #1327

Changes

• New functions accept a list of dimensions (e.g. [2,2,2] for 3D cube)
• periodic option for wrapping boundaries
• with_positions option to store coordinates as node weights

Example

cube = rx.generators.nd_grid_graph([2, 2, 2])  # 8 nodes, 12 edges
grid = rx.generators.nd_grid_graph([3, 3], with_positions=True)

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