message: Remove WAR of CombinedNMS converter that shrinks last output dim
  This WAR is still needed for <=TRT5.1
diff: @@ -5215,6 +5215,18 @@ Status ConvertCombinedNMS(OpConverterParams* params) {
  &plugin_inputs[0], static_cast<int>(plugin_inputs.size()), *plugin);
  TFTRT_RETURN_ERROR_IF_NULLPTR(layer, node_def.name());
  + // Set plugin outputs
  + nvinfer1::ITensor* output_nmsed_boxes = layer->getOutput(1);
  +#if IS_TRT_VERSION_GE(6, 0, 0, 0)
  + // TRT6 fixes (removes) the extra last dimension in CombinedNMS outputs
  + nvinfer1::ITensor* output_num_detections = layer->getOutput(0);
  + nvinfer1::ITensor* output_nmsed_scores = layer->getOutput(2);
  + nvinfer1::ITensor* output_nmsed_classes = layer->getOutput(3);
  +#else
  + nvinfer1::ITensor* output_num_detections = nullptr;
  + nvinfer1::ITensor* output_nmsed_scores = nullptr;
  + nvinfer1::ITensor* output_nmsed_classes = nullptr;
  +
  auto shrink_last_dim = [params](nvinfer1::ITensor* in_tensor,
  nvinfer1::ITensor** out_tensor) {
  nvinfer1::Dims dims = in_tensor->getDimensions();
  @@ -5228,18 +5240,13 @@ Status ConvertCombinedNMS(OpConverterParams* params) {
  /*validation_only=*/false, out_tensor));
  return Status::OK();
  };
  -
  - // Set plugin outputs
  - nvinfer1::ITensor* output_nmsed_boxes = layer->getOutput(1);
  - nvinfer1::ITensor* output_nmsed_scores = nullptr;
  - nvinfer1::ITensor* output_nmsed_classes = nullptr;
  - nvinfer1::ITensor* output_num_detections = nullptr;
  TF_RETURN_IF_ERROR(
  shrink_last_dim(layer->getOutput(2), &output_nmsed_scores));
  TF_RETURN_IF_ERROR(
  shrink_last_dim(layer->getOutput(3), &output_nmsed_classes));
  TF_RETURN_IF_ERROR(
  shrink_last_dim(layer->getOutput(0), &output_num_detections));
  +#endif // IS_TRT_VERSION_GE(6, 0, 0, 0)
  params->outputs->push_back(TRT_TensorOrWeights(output_nmsed_boxes));
  params->outputs->push_back(TRT_TensorOrWeights(output_nmsed_scores));
  