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Summary:
For non square image, the NDC space in pytorch3d is not square [-1, 1]. Instead, it is [-1, 1] for the smallest side, and [-u, u] for the largest side, where u > 1. This behavior is followed by the pytorch3d renderer.

See the function `get_ndc_to_screen_transform` for a example.

Without this fix, the rendering result is not correct using the converted pytorch3d-camera from a opencv-camera on non square images.

This fix also helps the `transform_points_screen` function delivers consistent results with opencv projection for the converted pytorch3d-camera.

Reviewed By: classner

Differential Revision: D31366775

fbshipit-source-id: 8858ae7b5cf5c0a4af5a2af40a1358b2fe4cf74b