fix bug in transformer_vae
fix bug in transformer_vae

PiperOrigin-RevId: 210459174

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