Allow optional optimizers part 1

apex/amp/_initialize.py

@@ -129,6 +129,8 @@ def _initialize(models, optimizers, properties):
     if isinstance(optimizers, torch.optim.Optimizer):
         optimizers_was_list = False
         optimizers = [optimizers]
+    elif optimizers is None:
+        optimizers = []
     elif isinstance(optimizers, list):
         optimizers_was_list = True
     else:
@@ -217,6 +219,12 @@ def _initialize(models, optimizers, properties):
             return models[0], optimizers
     else:
         if models_was_list:
+            if len(optimizers) == 0:
+                return models
+            else:
                 return models, optimizers[0]
+        else:
+            if len(optimizers) == 0:
+                return models[0]
             else:
                 return models[0], optimizers[0]