example: add dataset structure validation helper

[AKilalours]

Summary

This PR adds a lightweight helper script for validating the local structure of a nuScenes dataset directory.

Why

New users can run into dataset path or extraction issues before initializing the devkit. This script checks whether the expected metadata files exist under the selected dataset version directory.

Changes

• Added validate_dataset_structure.py
• Validates the existence of required nuScenes metadata files
• Supports configurable --dataroot and --version
• Provides clearer error messages for missing dataroot, version folder, or metadata files

Example

python python-sdk/nuscenes/scripts/validate_dataset_structure.py \
    --dataroot /data/sets/nuscenes \
    --version v1.0-mini

Testing

• Ran python -m py_compile on the new script
• Checked diff locally

[AKilalours]

Hi maintainers,

I opened this as a small scoped contribution to improve the beginner experience when working with the nuScenes mini dataset.

Happy to revise the file location, naming, or scope if you prefer a different structure.