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skeptical_lurker comments on "Human-level control through deep reinforcement learning" - computer learns 49 different games - Less Wrong
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Training networks layer by layer was the trend from the mid to late 2000s up until early 2012, but that changed in mid 2012 when Alex Krizhevsky and Geoff Hinton finally got neural nets to work for large-scale tasks in computer vision. They simply trained the whole network jointly with stochastic gradient descent, which has remained the case for most neural nets in vision since then.
Really? I was under the impression that training the whole network with gradient decent was impossible, because the propagated error becomes infinitesimally small. In fact, I thought that training layers individually was the insight that made DNNs possible.
Do you have a link about how they managed to train the whole network?
That was indeed one of the hypotheses about why it was difficult to train the networks - the vanishing gradient problem. In retrospect, one of the main reasons why this happened was the use of saturating nonlinearities in the network -- nonlinearities like the logistic function or tanh which asymptote at 1. Because they asymptote, their derivatives always end up being really small, and the deeper your network the more this effect compounds. The first large-scale network that fixed this was by Krizhevsky et al., which used a Rectified Linear Unit (ReLU) for their nonlinearity, given by f(x) = max(0, x). The earliest reference I can find to using ReLUs is Jarrett et al., but since Krizhevsky's result pretty much everyone uses ReLUs (or some variant thereof). In fact, the first result I've seen showing that logistic/tanh nonlinearities can work is the batch normalization paper Seanoh linked, which gets around the problem by normalizing the input to the nonlinearity, which presumably prevents the units from saturating too much (though this is still an open question).
If you do it naively, yes. But researches figured out how to attack that problem from multiple angles: from the choice of the non-linear activation function, to specifics of the optimization algorithm, to the random distribution used to sample the initial weights.
The batch normalization paper cited above is one example of that.