support for TF frozen models in model inference

[franperezlopez]

please consider adding support for frozen models when using the model inference tool (given that this support is already present in the model converter)

[linmajia]

@franperezlopez , thank you very much for the feedback.

Currently, we support TensorFlow checkpoint and saved model only. To export to SavedModel format, there is an example at https://github.com/Microsoft/samples-for-ai/blob/master/projects/StyleTransfer/StyleTransferTraining/src/train.py#L218 or you could simply call tf.saved_model.simple_save.