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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def num_trees(self):
 """Get number of weak sub-models. Returns ------- num_trees : int The number of weak sub-models. """

        num_trees = ctypes.c_int(0)
        _safe_call(_LIB.LGBM_BoosterNumberOfTotalModel(
            self.handle,
            ctypes.byref(num_trees)))
        return num_trees.value