fix: bug in vgg16_fp8_ptq example

[zewenli98]

Description

Fix a bug in vgg16_fp8_ptq.py example.

Fixes #2932

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ ] My code follows the style guidelines of this project (You can use the linters)
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas and hacks
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests to verify my fix or my feature
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have added the relevant labels to my PR in so that relevant reviewers are notified