replace object detect model

[yang LU]

In object detection

replace ssdlite_object_detection.tflite with ssd_mobilenet_v2.
the problem can be reproduced by this:

• use the gprah "mediapipe/graphs/object_detection/object_detection_desktop_live.pbtxt",
• use the model SSDMobileNet-V2 (float 32), the model is download from your website, but also have the same problem.
• In pbtxt set image size: 256, num_boxes: 12276.
  and finally no errors, but detections are wrong

Can anybody help me? I'm stuck for several days