configurable `distribute_reduction_method` in Model. additionally allows to reduce with `sum` to account for the metrics/losses divided by global batch size -  when training using custom training step and MirroredStrategy
configurable `distribute_reduction_method` in Model.
additionally allows to reduce with `sum` to account for the metrics/losses divided by global batch size -  when training using custom training step and MirroredStrategy

keras-team/keras,keras-team/keras