Issues in Pascal Context GT Visualization

Steps to Reproduce the Bug:

1. Full runnable code or changes made:
Refer to the failed pull request: #5391

2. Exact command run:
I executed `tools/visualize_data` and noticed that `visualize.py` in the `utils` folder uses an outdated numpy dtype.

3. Logs or relevant observations:
The image should correspond to train(id: 359), but there is an overflow issue, referencing fly(id: 359-256 = 103) instead.

**Expected Behavior:**
It should handle classes greater than 256, but Pascal Context 459 results in an overflow.

**Environment:**
- Python 3.10
- NumPy 1.22.4
- PyTorch 1.13.1

Run tools/visualize_data and inspect utils/visualize.py, focusing on the outdated NumPy dtype involved in class handling. Verify the visualization for Pascal Context 459, including train(id: 359), and confirm that classes greater than 256 no longer overflow or map to fly(id: 103).

Written by the indexing model from the issue text.