Fixes to mixture-of-experts code for mtf-transformer2 - feed in a tensor that indicates nonpadding positions.
Fixes to mixture-of-experts code for mtf-transformer2 - feed in a tensor that indicates nonpadding positions.

PiperOrigin-RevId: 233684322

tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor