Add bool function to return whether a model contains loss

Summary:
Add a function to return true if the model contains loss and retuen
false if the model doesn't include a loss.

Reviewed By: kittipatv

Differential Revision: D6982444

fbshipit-source-id: 1f63b7a1eaa3077841a0ad5d8d854b471d0aa84c

caffe2/python/layer_model_helper.py

@@ -374,6 +374,9 @@ def loss(self, loss):
         assert self._loss is None
         self._loss = loss
 
+    def has_loss(self):
+        return self._loss is not None
+
     def add_loss(self, loss, name='unnamed'):
         assert loss is not None, "Added loss should not be None"
         assert isinstance(loss, schema.Scalar) or isinstance(