[Numpy Extension] Add categorical sampling to npx

[sxjscience]

We should provide an API that offers the same functionality as https://www.tensorflow.org/api_docs/python/tf/random/categorical in the numpy interface.

In fact, we have this functionality in the legacy mx.nd namespace, i.e.,mx.nd.sample_multinomial: See https://github.com/apache/incubator-mxnet/blob/eceb5f2c1c494094c1a697286f2c4560b7ca472e/src/operator/random/sample_multinomial_op.cc#L33

One straightforward extension is to register this op as mx.npx.random.categorical.

@yzhliu