title: randomly sample from a high dimensional array along with a specific dimension
body: <p>There has a 3-dimensional array x of shape <code>(2000,60,5)</code>. If we think it represents a video, the 2000 can represent <code>2000</code> frames. I would like to randomly sample it along with the first dimension, i.e., get a set of frame samples. For instance, how to get an array of <code>(500,60,5)</code> which is randomly sampled from <code>x</code> along with the first dimension?</p>