[Cherry pick] rename BoundingBoxes module to match class name (#7910) (#7918)

Co-authored-by: Philip Meier <github.pmeier@posteo.de>

torchvision/tv_tensors/__init__.py

 import torch
 
-from ._bounding_box import BoundingBoxes, BoundingBoxFormat
+from ._bounding_boxes import BoundingBoxes, BoundingBoxFormat
 from ._image import Image
 from ._mask import Mask
 from ._torch_function_helpers import set_return_type