fix tf.while bug when using eval_run_autoregressive.
fix tf.while bug when using eval_run_autoregressive.

PiperOrigin-RevId: 171345135

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