corrected pairing in SSD head of pointpillars

[zhongyidu]

Accordiing to the PointPillars paper, "A positive match is either the highest with a ground truth box, or above the positive match threshold". While the latter point is implemented correctly, there's a minor mistake in the implementation of the first point. The original code only updates the bool array pos_idx without changing the matched target to the correct ground truth index. This could lead to ground truth box without a valid match in the end.

When I corrected the SSD matching, I also referred to this implementation of SSD: https://github.com/amdegroot/ssd.pytorch/blob/5b0b77faa955c1917b0c710d770739ba8fbff9b7/layers/box_utils.py#L106

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This change is 

[benjaminum]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed (commit messages unreviewed), all discussions resolved (waiting on @zhongyidu)

[zhongyidu]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @zhongyidu)