Return which inputs of this operation are variable (i.e. depend on the model inputs).

def _variable_inputs(self, op):
        """ Return which inputs of this operation are variable (i.e. depend on the model inputs).
        """
        if op.name not in self._vinputs:
            self._vinputs[op.name] = np.array([t.op in self.between_ops or t in self.model_inputs for t in op.inputs])
        return self._vinputs[op.name]