Add https://youtu.be/96NkhsV-W1U to docs and heatmaps fix (#7944)

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

ultralytics/solutions/heatmap.py

@@ -167,9 +167,10 @@ class Heatmap:
         """
         self.im0 = im0
         if tracks[0].boxes.id is None:
+            self.heatmap = np.zeros((int(self.imh), int(self.imw)), dtype=np.float32)
             if self.view_img and self.env_check:
                 self.display_frames()
-            return
+            return im0
         self.heatmap *= self.decay_factor  # decay factor
         self.extract_results(tracks)
         self.annotator = Annotator(self.im0, self.count_txt_thickness, None)