Fix rrpn proposal validation
Fix rrpn proposal validation

Summary:
https://github.com/facebookresearch/detectron2/blob/08f617cc2c9276a48e7e7dc96ae946a5df23af3f/detectron2/modeling/proposal_generator/proposal_utils.py#L104
```python
        if not valid_mask.all():
            if training:
                raise FloatingPointError(
                    "Predicted boxes or scores contain Inf/NaN. Training has diverged."
                )
            boxes = boxes[valid_mask]
            scores_per_img = scores_per_img[valid_mask]
            lvl = lvl[valid_mask]
```

Unlike function **find_top_rpn_proposals**, **find_top_rrpn_proposals** does not include valid masking for level indices.
When the program goes into if condition, it gives an undesirable error says that "shape does not match".

I add a handling code same with **find_top_rpn_proposals**.

Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/3770

Reviewed By: sstsai-adl

Differential Revision: D33119548

Pulled By: zhanghang1989

fbshipit-source-id: fee677ad6b610deb713d2769901ab8d3d47539c3

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2