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In the lastest version of lightning with DDP training, If I use validation_step_outputs to store validation outputs, how to compute a custom metric in the on_validation_epoch_end function, I now do it with clear in on_validation_epoch_start and compute metric in the on_validation_epoch_end, but in this way, the compute function will be executed n_GPUs times, how can I do the compute function only once？
Thanks for any kind help.

cc @lantiga @Borda