ML.NET 1.3.1

Deep Neural Networks Training (PREVIEW) (#4057) Introduces in-preview 0.15.1 Microsoft.ML.DNN package that enables full DNN model retraining and transfer learning in .NET using C# bindings for tensorflow provided by Tensorflow .NET. The goal of this package is to allow high level DNN training and scoring tasks such as image classification, text classification, object detection, etc using simple yet powerful APIs that are framework agnostic but currently they only uses Tensorflow as the backend. The below APIs are in early preview and we hope to get customer feedback that we can incorporate in the next iteration.