message: Clarifying the docstring for how gradients are reduced across towers in replicate_model_fn
diff: @@ -110,7 +110,8 @@ def replicate_model_fn(model_fn,
  Certain algorithms were chosen for aggregating results of computations on
  multiple towers:
  - Losses from all towers are reduced according to `loss_reduction`.
  - - Gradients are reduced using sum for each trainable variable.
  + - Gradients from all towers are reduced according to `loss_reduction`
  + for each trainable variable.
  - `eval_metrics_ops` are reduced per metric using `reduce_mean`.
  - `EstimatorSpec.predictions` and `EstimatorSpec.export_outputs` are
  reduced using concatenation.
  