add backwards compatibility for PyTorch 0.4 for named_buffers (#331)

apex/amp/_initialize.py

@@ -89,7 +89,16 @@ def check_params_fp32(models):
                         "you chose. Use model.to('cuda') to use the default device.".format(
                         name, param.type()))
 
-        for name, buf in model.named_buffers():
+        # Backward compatibility for PyTorch 0.4
+        if hasattr(model, 'named_buffers'):
+            buf_iter = model.named_buffers()
+        else:
+            buf_iter = model._buffers
+        for obj in buf_iter:
+            if type(obj)==tuple:
+                name, buf = obj
+            else:
+                name, buf = obj, buf_iter[obj]
             if buf.is_floating_point():
                 if 'Half' in buf.type():
                     warn_or_err("Found buffer {} with type {}, expected torch.cuda.FloatTensor.\n"