Fix shape mismatch issue when target sequence length > logits sequence length
Fix shape mismatch issue when target sequence length > logits sequence length

waterblue13/tensor2tensor,vthorsteinsson/tensor2tensor,rsepassi/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor,rsepassi/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor,waterblue13/tensor2tensor,tensorflow/tensor2tensor,waterblue13/tensor2tensor,rsepassi/tensor2tensor,vthorsteinsson/tensor2tensor,rsepassi/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor