Give custom to method a higher priority (#364)

apex/amp/_initialize.py

@@ -40,12 +40,12 @@ def applier(value, fn):
         return value
     elif isinstance(value, np.ndarray):
         return value
+    elif hasattr(value, "to"): # Allow handling of custom batch classes
+        return fn(value)
     elif isinstance(value, container_abcs.Mapping):
         return {applier(k, fn) : applier(v, fn) for k, v in value.items()}
     elif isinstance(value, container_abcs.Iterable):
         return type(value)(applier(v, fn) for v in value)
-    elif hasattr(value, "to"): # Allow handling of custom batch classes
-        return fn(value)
     else:
         # Do I want this to fire off even if someone chooses to pass something ordinary like
         # an int or float?  May be more annoying than it's worth.