avoid list[ndarray] -> tensor conversion in Boxes
avoid list[ndarray] -> tensor conversion in Boxes

Summary:
avoid wanings like:
```
detectron2/structures/boxes.py:158: UserWarning: Creating a tensor from a list
of numpy.ndarrays is extremely slow. Please consider converting the list to a single numpy.ndarray
with numpy.array() before converting to a tensor. (Triggered internally at torch/csrc/utils/tensor_new.cpp:201.)
```

Reviewed By: wat3rBro

Differential Revision: D30172338

fbshipit-source-id: fbe044fead72722b3c6eb9916faa2a7dca3e6eae

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2