Fix early stopping off by 2 (min_epochs) (#617)

* fix early stopping off by 2

* add min_epochs example in docs

Author: Adrian Wälchli

pytorch_lightning/trainer/training_loop.py

@@ -50,6 +50,9 @@
     # pass in your own to override the default callback
     trainer = Trainer(early_stop_callback=early_stop_callback)
 
+    # pass in min_epochs to enable the callback after min_epochs have run
+    trainer = Trainer(early_stop_callback=early_stop_callback, min_epochs=5)
+
     # pass in None to disable it
     trainer = Trainer(early_stop_callback=None)
 
@@ -339,7 +342,7 @@ def train(self):
                 self.reduce_lr_on_plateau_scheduler.step(val_loss, epoch=self.current_epoch)
 
             # early stopping
-            met_min_epochs = epoch > self.min_epochs
+            met_min_epochs = epoch >= self.min_epochs - 1
             if self.enable_early_stop and (met_min_epochs or self.fast_dev_run):
                 should_stop = self.early_stop_callback.on_epoch_end(epoch=epoch,
                                                                     logs=self.callback_metrics)