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Summary:
The `chamfer_distance` function currently allows `"sum"` or `"mean"` reduction, but does not support returning unreduced (per-point) loss terms. Unreduced losses could be useful if the user wishes to inspect individual losses, or perform additional modifications to loss terms before reduction. One example would be implementing a robust kernel over the loss.

This PR adds a `None` option to the `point_reduction` parameter, similar to `batch_reduction`. In case of bi-directional chamfer loss, both the forward and backward distances are returned (a tuple of Tensors of shape `[D, N]` is returned). If normals are provided, similar logic applies to normals as well.

This PR addresses issue https://github.com/facebookresearch/pytorch3d/issues/622.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1605

Reviewed By: jcjohnson

Differential Revision: D48313857

Pulled By: bottler

fbshipit-source-id: 35c824827a143649b04166c4817449e1341b7fd9