Building Keras from source: A follow along guide

Keras has recently taken a big step towards improving the developer experience by hosting the codebase in a separate repository. As mentioned in the RFC, one of the main objectives is to eliminate the lengthy feedback loop caused due to the long build times of the core TensorFlow library. Due to this change, it is now possible to run tests in an extremely feasible amount of time. This blog post aims to be a do-it-along guide for Keras' building and testing procedure. To all the new ones here, I'd like to take a moment to explain what "building from source" exactly means.