question: How to disable the automatic reduce/mean while using dp?
context: Hello everyone,
  I have upgraded pytorch-lightning to 1.2.6 recently, the behavior of dp seems different from 1.2.0. To be specific, the returned values of validation_step() are automatically reduced before sent to validation_epoch_end(). However, the metrics I use need the original predictions of each sample instead of the reduced values. Is there a way to disable the automatic reduce and pass the whole predictions to validation_epoch_end()? Note that the validation_step_end() is not implemented in my model.
  cc: @tchaton
answer: I notice that the validation_step_end() and test_step_end() functions in dp.py script are:
  def validation_step_end(output):
         return self.reduce(output)
  
  def test_step_end(output):
         return self.reduce(output)
  
  Thus, overwrite these two methods as follows will disable the automatic reduce in evaluation and test:
  def validation_step_end(output):
         return output
  
  def test_step_end(output):
         return output