Bugs in Pascal Context GT visualization

## Instructions To Reproduce the 🐛 Bug:
1. Full runnable code or full changes you made:

details can refer to the failed pull requests: https://github.com/facebookresearch/detectron2/pull/5391

2. What exact command you run:
I run the tools/visualize_data and find the visualize.py in utils have an out of date numpy dtype.

4. __Full logs__ or other relevant observations:

this should be train(id:359) howver is overflow and refer to fly(id:359-256=103)

5. please simplify the steps as much as possible so they do not require additional resources to
run, such as a private dataset.

expect to handle classes more than 256, however Pascal Context 459 is overflow.

## Environment:

Python 3.10 numpy 1.22.4 torch 1.13.1

Start by running tools/visualize_data and inspect the visualize.py utility mentioned in the report, using the Pascal Context example and the stated Python, NumPy, and PyTorch versions. Trace the dtype used for class IDs and verify that classes above 256, including train (id 359), are rendered with the correct label rather than overflowing to fly (id 103).

Written by the indexing model from the issue text.