update model initialization design, supports custom data/num_classes (#44)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ultralytics/yolo/utils/downloads.py

@@ -140,8 +140,3 @@ def download(url, dir=Path.cwd(), unzip=True, delete=True, curl=False, threads=1
     else:
         for u in [url] if isinstance(url, (str, Path)) else url:
             download_one(u, dir)
-
-
-def get_model(model: str):
-    # check for local weights
-    pass