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Indicates how a distributed variable will be aggregated.
tf.distribute.Strategy distributes a model by making multiple copies
(called "replicas") acting data-parallel on different elements of the input
batch. When performing some variable-update operation, say
var.assign_add(x), in a model, we need to resolve how to combine the
different values for
x computed in the different replicas.
NONE: This is the default, giving an error if you use a variable-update operation with multiple replicas.
SUM: Add the updates across replicas.
MEAN: Take the arithmetic mean ("average") of the updates across replicas.
ONLY_FIRST_REPLICA: This is for when every replica is performing the same update, but we only want to perform the update once. Used, e.g., for the global step counter.