Adjust NonMaxSuppression score_threshold in build_detection_graph for ssd networks

[cosama]

It took me quite some time to figure this out, so I think people would be happy to have this automatically done for them when running the build_detection_graph function. The threshold can be adjusted by passing in an argument. With this fix, it should be possible to get the described performance without any tweaks to the models.

I used the warnings module to inform the user about if the value was changed or if it was not possible to change it. I can change that if necessary.

[ghost]

Thanks for sharing this. I agree that being able to easily set the NMS score threshold is a helpful feature. We are considering adding this, as well as some other changes to improve model support.

I share your concern that defaulting to a threshold of 0.3 may be surprising (especially if users modify their configuration directly).

Since others have shared your frustration though, I will add a note to the README to suggest modifying the score threshold directly, which will hopefully ease some pain.

[cosama]

I'm not too frustrated with it, just took me some time to figure out where to change that parameter. I think it would be nice for others to have that implemented right away and get what they are promised without much work.

I saw that you have added this to the Todo list in the improved_model_support branch. I can either close this for now or leave it open for later use, whatever you prefer?

[ghost]

I think it's okay to leave it open for now. I will close if we merge or implement an alternative. Thanks.