Adds support for YOLOv12 model upload

[lrosemberg]

Description

Adds support for YOLOv12 model upload.

For YOLOv12 I have changed the approach.

• roboflow-python is no longer responsible for extracting the state_dict and model_artifacts
• now this is done here in model conversion
• roboflow-python no longer needs the dependency on ultralytics to upload yolov12 models.

Type of change

• New feature (non-breaking change which adds functionality)

How has this change been tested, please provide a testcase or example of how you tested the change?

• Staging

Any specific deployment considerations

• Release version

[tonylampada]

good stuff!