changed _normalize_body_output to sum the mean of each loss when loss… (#1383)
changed _normalize_body_output to sum the mean of each loss when loss… (#1383)

* changed _normalize_body_output to sum the mean of each loss when loss is a list

* generalized for tuples and tensor-like losses

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