Simplify numpy-based top-k (#340)
Simplify numpy-based top-k (#340)

Modify the `np.argpartition` call to return an already sorted top-k to
avoid the extra sort.
awslabs/sockeye,artemsok/sockeye,awslabs/sockeye,awslabs/sockeye,artemsok/sockeye,artemsok/sockeye