[T2T] Fixes a bug that resulted in incorrect TPU summaries on tensorboard.
[T2T] Fixes a bug that resulted in incorrect TPU summaries on tensorboard.

Description: At the moment, training and eval summaries are written out to the same directory. This can manifest itself in a number of ways:

1) Tensorboard incorrectly truncates training summaries if the corresponding summary is not present in eval.
2) Tensorboard mixes the losses from train and eval and labels them both as 'train'. This can cause the training loss to appear jagged.

PiperOrigin-RevId: 260207502

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