Based on the information provided above, looks like you are trying to convert a model from Tensorflow Object Detection API. When we checked the onnx file, and as far as we can tell, it looks to be an SSD Mobilenet V2 320x320 model.
Please refer TensorRT/samples/python/tensorflow_object_detection_api. This repo helps in converting most of the models trained via TF OD.
This work is still in progress, and soon will be available in TRT OSS repo. As of now it is a private repo, you can DM me to provide the access.
Depending on how we trained a network, it is highly recommended to use TFOD export script. The next step would be using create_onnx.py from above shared project, if it is SSD Mobilenet V2 320x320 model sample command would be
python create_onnx.py --pipeline_config /path_to/pipeline.config --saved_model /path_to/saved_model --onnx /dir_to_save/model.onnx --batch_size 1
This should automatically build ONNX if model is supported.
The next step would be to build_engine.py, sample command is
python build_engine.py --onnx /dir_to_saved/model.onnx --engine /dir_to_save/engine.trt --precision fp32 --verbose
Thank you.