[MNIST] Implement minibatch gradient descent and print accuracy. (#21)
[MNIST] Implement minibatch gradient descent and print accuracy. (#21)

* Implement minibatch gradient descent.
  * Todo: randomly sample minibatches using TensorFlow dataset APIs.
* Print accuracy (rather than loss) to clearly show that the model is improving.
tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models