* tensorflow or tflite exclusively as interpreter

As per bug report https://github.com/ultralytics/yolov5/issues/5709 I think there should be only one attempt to assign interpreter, and it appears tflite is only ever needed for the case of edgetpu model.

* Scope imports

* Nested definition line fix

* Update common.py
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>