--resume training from URL weights (ultralytics#8882)

@kalenmike

train.py

@@ -485,9 +485,8 @@ def main(opt, callbacks=Callbacks()):
         check_requirements(exclude=['thop'])
 
     # Resume
-    if opt.resume and not (check_wandb_resume(opt) or opt.evolve):  # resume an interrupted run
-        last = Path(opt.resume if isinstance(opt.resume, str) else get_latest_run())  # specified or most recent last.pt
-        assert last.is_file(), f'ERROR: --resume checkpoint {last} does not exist'
+    if opt.resume and not (check_wandb_resume(opt) or opt.evolve):  # resume from specified or most recent last.pt
+        last = Path(check_file(opt.resume) if isinstance(opt.resume, str) else get_latest_run())
         opt_yaml = last.parent.parent / 'opt.yaml'  # train options yaml
         opt_data = opt.data  # original dataset
         if opt_yaml.is_file():