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Summary:
- Old NDC convention had xy coords in [-1,1]x[-1,1]
- New NDC convention has xy coords in [-1, 1]x[-u, u] or [-u, u]x[-1, 1]

where u > 1 is the aspect ratio of the image.

This PR fixes the NDC raysampler to use the new convention.

Partial fix for https://github.com/facebookresearch/pytorch3d/issues/868

Pull Request resolved: https://github.com/fairinternal/pytorch3d/pull/29

Reviewed By: davnov134

Differential Revision: D31926148

Pulled By: bottler

fbshipit-source-id: c6c42c60d1473b04e60ceb49c8c10951ddf03c74