In Chapter 10 we introduced artificial neural networks and trained our first deep neural networks. But they were shallow nets, with just a few hidden layers. What if you need to tackle a complex problem, such as detecting hundreds of types of objects in high-resolution images? You may need to train a much deeper DNN, perhaps with 10 layers or many more, each containing hundreds of neurons, linked by hundreds of thousands of connections. Training a deep DNN isn’t a walk in the park. Here are some of the problems you could run into:
You may be faced with the tricky vanishing gradients problem or the related exploding gradients problem. This is when the gradients grow smaller and smaller, or larger and larger, when flowing backward through the DNN during training. Both of these problems make lower layers very hard to train.
You might not have enough training data for such a large network, or it might be too costly to label.
Training may be extremely slow.
A model with millions of parameters would severely risk overfitting the training set, especially if there are not enough training instances or if they are too noisy.
In this chapter we will go through each of these problems and present techniques to solve them. We will start by exploring the vanishing and exploding gradients problems and some of their most popular solutions. Next, we will look at transfer learning and unsupervised pretraining, which can help you tackle complex ...