Fixed NMS error when outputting to Keras (.h5)

[PINTO0309]

1. Content and background

• Fixed NMS error when outputting to Keras (.h5)
• https://s3.ap-northeast-2.wasabisys.com/temp-models/onnx2tf_146/yolox_x.onnx
• https://github.com/PINTO0309/onnx2tf/releases/download/1.1.28/faster_rcnn-10.onnx
• https://github.com/PINTO0309/onnx2tf/releases/download/1.1.28/yolov7_tiny_head_0.768_post_480x640.onnx

  onnx2tf -i yolox_x.onnx -oh5
  

  ValueError: This Keras op layer was generated from <function non_max_suppression at 0x7fd6a96ff1f0>,
  a method that is not publicly exposed in the TensorFlow API.
  This may have happened if the method was explicitly decorated to add dispatching support,
  and it was used during Functional model construction.
  To ensure cross-version compatibility of Keras models that use op layers,
  only op layers produced from public TensorFlow API symbols can be serialized.
  

• Define NMS layers to avoid problems
  

2. Summary of corrections

3. Before/After (If there is an operating log that can be used as a reference)

4. Issue number (only if there is a related issue)

[YOLOX-X] Error when outputting to h5 file. AttributeError: 'numpy.dtype[int64]' object has no attribute 'item' #146
[YOLOX-X] TensorFlow aborts when exporting a model with NMS to Keras (.h5) #150