Fix 'local variable 'optimizers_was_list' referenced before assignment' when...

Fix 'local variable 'optimizers_was_list' referenced before assignment' when amp.initialize() is called with optimizers=None (#218)

apex/amp/_initialize.py

@@ -126,8 +126,8 @@ def _initialize(models, optimizers, properties):
     from apex.parallel import DistributedDataParallel as apex_DDP
     from .amp import init as amp_init
 
-    if isinstance(optimizers, torch.optim.Optimizer):
     optimizers_was_list = False
+    if isinstance(optimizers, torch.optim.Optimizer):
         optimizers = [optimizers]
     elif optimizers is None:
         optimizers = []