I am guessing this has something to do with not specifying the input for  model-runner  (the way we do, for instance, with  image-classifier ). But then again, there is no way to specify input for  model-runner .

Yeah sorry for the confusion – you’ve diagnosed the problem correctly. As I noted in my other reply in the other thread 1, the ModelRunner is more of a toy, and your model must not have any inputs, and exactly one output.
I would suggest creating your own model loader/runner, perhaps initially based on ModelRunner since it’s the simplest, which is customized for your model. It would have a single input with name input_data and a single output named output. You can look at ImageClassifier to see how it creates its Caffe2ModelLoader/ONNXModelLoader with inputName along with a Tensor Type inputImageType, and then calls updateInputPlaceholders() to update the Tensor for the input before running.