Fix bboxes being overwritten before use in prepare_example (#50218)

Rename variable 'bboxes' to 'in_bboxes' in example to Fix `bboxes` being overwritten before use in `prepare_example`

**Problem**
In `prepare_example`, `objects["bbox"]` is assigned to `bboxes`, and a few lines
later `bboxes, cats, areas, ids = [], [], [], []` rebinds the same name to an
empty list. `zip(bboxes, categories)` then iterates over nothing, so every example
is returned with an empty `bbox` list and the `filter(len(...) > 0)` step drops
the whole dataset (train/validation sizes print as 0).
**Check**
After the change, `Train: <N>, Validation: <M>` prints non-zero sizes.

Author: Jose-Marcial-GF

docs/source/en/tasks/object_detection.md

@@ -87,11 +87,11 @@ categories to integer ids, compute areas, and filter out degenerate bounding box
 ```py
 >>> def prepare_example(example, idx):
 ...     objects = example["objects"]
-...     bboxes = objects["bbox"]
+...     in_bboxes = objects["bbox"]
 ...     categories = objects["category"]
 ...     img_w, img_h = example["width"], example["height"]
 ...     bboxes, cats, areas, ids = [], [], [], []
-...     for i, (bbox, cat) in enumerate(zip(bboxes, categories)):
+...     for i, (bbox, cat) in enumerate(zip(in_bboxes, categories)):
 ...         x, y, w, h = bbox
 ...         if w <= 0 or h <= 0:
 ...             continue