Add model graph to summary produced by model_main_tf2.py so it can be inspected in TensorBoard

Prerequisites

Please answer the following question for yourself before submitting an issue.

• I checked to make sure that this feature has not been requested already.

1. The entire URL of the file you are using

https://github.com/tensorflow/models/blob/a8eae76f768edb3949689f8593c851c37cf689dd/research/object_detection/model_main_tf2.py

2. Describe the feature you request

When training a model using model_main_tf2.py it is not possible to view and inspect the model using TensorBoard. It would be desirable to have have this functionality brought to TensorFlow 2 object detection models, similar to how it is done through model_main.py for TensorFlow 1 models.

3. Additional context

Add any other context about the feature request here.

4. Are you willing to contribute it? (Yes or No)

Yes