Add an hparam to sample a random top-k rather than always the nearest neighbor.
Add an hparam to sample a random top-k rather than always the nearest neighbor.

PiperOrigin-RevId: 181234215

vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,rsepassi/tensor2tensor,vthorsteinsson/tensor2tensor,rsepassi/tensor2tensor,rsepassi/tensor2tensor,rsepassi/tensor2tensor,vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor