feat: add webcam support to NanoDet demo

[ojayballer]

Motivation

Currently, the NanoDet demo script crashes when providing a webcam index (e.g., --input 0) because it attempts to load "0" as an image file.

Modification

This PR adds input validation to demo.py:

• Checks if args.input is a digit using .isdigit().
• If it is a digit, it initializes the webcam (cv.VideoCapture).
• If it is not, it proceeds with the existing image file loading logic.

Verification

• Tested with webcam: python demo.py --input 0 (Successful inference)
• Tested with image: python demo.py --input dog.jpg (Backwards compatibility preserved)