Allow checkpoint_path to be an url (Required for transfer learning)

[BloodAxe]

• Allow checkpoint_path to be an url (Required for transfer learning)
• Added synchronization primitives to prevent race conditions when downloading weights

One can now load checkpoints like so:

models.get(Models.YOLO_NAS_S, num_classes=80, checkpoint_path="https://sghub.deci.ai/models/yolo_nas_s_coco.pth")

Why we want this to have: For training YOLO-NAS-POSE we want to use YOLO-NAS-OD pretrained weights like so:

checkpoint_params:
  checkpoint_path: "https://sghub.deci.ai/models/yolo_nas_s_coco.pth"

This PR enables us to do this.

[Louis-Dupont]

LGTM, just small question below