Fix counter overflow for video frame crop names, set default separator for same-frame detection counter (#3982)

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/engine/predictor.py

@@ -105,6 +105,7 @@ class BasePredictor:
         self.results = None
         self.transforms = None
         self.callbacks = _callbacks or callbacks.get_default_callbacks()
+        self.txt_path = None
         callbacks.add_integration_callbacks(self)
 
     def get_save_dir(self):
@@ -178,7 +179,8 @@ class BasePredictor:
         if self.args.save_txt:
             result.save_txt(f'{self.txt_path}.txt', save_conf=self.args.save_conf)
         if self.args.save_crop:
-            result.save_crop(save_dir=self.save_dir / 'crops', file_name=self.data_path.stem)
+            result.save_crop(save_dir=self.save_dir / 'crops',
+                             file_name=self.data_path.stem + ('' if self.dataset.mode == 'image' else f'_{frame}'))
 
         return log_string