NameError: name 'mpu' is not defined (#305)

mpu object is bound to the class instance.. 

the if statement uses  `self.mpu'  but just `mpu` is called in the following lines.. 

This raises a NameError

deepspeed/pt/deepspeed_light.py

@@ -650,8 +650,8 @@ class DeepSpeedLight(Module):
         data_parallel_world_size = None
         data_parallel_rank = None
         if self.mpu is not None:
-            data_parallel_world_size = mpu.get_data_parallel_world_size()
-            data_parallel_rank = mpu.get_data_parallel_rank()
+            data_parallel_world_size = self.mpu.get_data_parallel_world_size()
+            data_parallel_rank = self.mpu.get_data_parallel_rank()
 
         return DeepSpeedDataLoader(dataset=dataset,
                                    batch_size=batch_size,