Structure for outputs returned from clients during federated optimization.

weights_delta A dictionary of updates to the model's trainable variables.
weights_delta_weight Weight to use in a weighted mean when aggregating weights_delta.
model_output A structure matching tff.learning.models.VariableModel.report_local_unfinalized_metrics, reflecting the results of training on the input dataset.
optimizer_output Additional metrics or other outputs defined by the optimizer.