Run on_validation_end only on main process in DDP (#1125)
Run on_validation_end only on main process in DDP (#1125)

Co-authored-by: xingzhaolee <c5c448fde01ffdd1e9312e6854bcb3ee062956ca@users.noreply.github.com>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning