Fix _KERAS_MERGE_LAYERS is undefined name
Fix _KERAS_MERGE_LAYERS is undefined name

_KERAS_MERGE_LAYERS is defined in _topology2 which is a conditional import based on _HAS_KERAS2_TF.  Without this change, an NameError might be raised at runtime.
apple/coremltools,apple/coremltools,apple/coremltools,apple/coremltools