title: Tensorflow indexing with boolean tensor
body: <p>In numpy, with two arrays of the same shape, <code>x</code> and <code>y</code>, it is possible to do slices like this <code>y[x &gt; 1]</code>. How do you achieve the same result in tensorflow? <code>y[tf.greater(x, 1)]</code> doesn't work and <code>tf.slice</code> doesn't support anything like this either. Is there a way to index with a boolean tensor right now or is that currently unsupported?</p>