Fix incorrect handling of on_batch_end edge cases in run_training_batch (#509)

* Fix returning only 2 values on an early exit. 

This fixes a bug 

`ValueError: not enough values to unpack (expected 3, got 2)`

* Update train_loop_mixin.py

* Change to return dict

The return value was actually a dict even though that variable is initialized as a list.

pytorch_lightning/trainer/train_loop_mixin.py

@@ -155,7 +155,7 @@ class TrainerTrainLoopMixin(object):
         all_log_metrics = []
 
         if batch is None:
-            return 0, grad_norm_dic
+            return 0, grad_norm_dic, {}
 
         # hook
         if self.is_function_implemented('on_batch_start'):
@@ -163,7 +163,7 @@ class TrainerTrainLoopMixin(object):
             response = model_ref.on_batch_start(batch)
 
             if response == -1:
-                return -1, grad_norm_dic
+                return -1, grad_norm_dic, {}
 
         splits = [batch]
         if self.truncated_bptt_steps is not None: