Voxel Downsample for Tensor interface

[theNded]

This PR addresses #6172 #5519 and allows averaging properties in voxel downsample. Previously, points are simply put at grid corners.

To achieve this, the operation IndexAdd_ (similar for pytorch) for tensors is introduced.

An alternative would be reusing Benjamin's Voxel Pooling. We may revisit this later.

Now the behavior is the same as legacy, but there is a slight voxel coordinate offset due to implementation discrepancies. The legacy version computes a bound that slightly shifts the voxels.

Next TODOs

• Further enhance backward compatibility via fixing discrepancies to legacy voxel downsample
• More tests for IndexAdd_
• Some screenshots

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[benjaminum]

Reviewable status: 0 of 11 files reviewed, 1 unresolved discussion (waiting on @ssheorey, @theNded, and @yxlao)

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cpp/pybind/t/geometry/pointcloud.cpp line 220 at r2 (raw file):

            "Downsamples a point cloud with a specified voxel size and a "
            "reduction type.",
            "voxel_size"_a, "reduction"_a = "mean");

Please update the documentation in google format and add a minimal example. See compute_convex_hull as an example.
Thank you!!! 😇

[theNded]

Reviewable status: 0 of 11 files reviewed, 1 unresolved discussion (waiting on @ssheorey, @theNded, and @yxlao)

cpp/pybind/t/geometry/pointcloud.cpp line 220 at r2 (raw file):

            "Downsamples a point cloud with a specified voxel size and a "
            "reduction type.",
            "voxel_size"_a, "reduction"_a = "mean");

Please update the documentation in google format and add a minimal example. See compute_convex_hull as an example. Thank you!!! innocent

Done. We could also use some time (and maybe GPT) to polish/fix the docs, including

• Overloaded initializers;
• Default parameters (esp. tensors);
• Always follow param: Description and fix the incorrect param. Description.

[benjaminum]

Thank you for the docstring fixes!

Reviewable status: 0 of 14 files reviewed, 1 unresolved discussion (waiting on @ssheorey and @yxlao)