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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 attach_grad(self, grad_req='write', stype=None):
 """Attach a gradient buffer to this NDArray, so that `backward` can compute gradient with respect to it. Parameters grad_req : {'write', 'add', 'null'} How gradient will be accumulated. - 'write': gradient will be overwritten on every backward. - 'add': gradient will be added to existing value on every backward. - 'null': do not compute gradient for this NDArray. stype : str, optional The storage type of the gradient array. Defaults to the same stype of this NDArray. """

        from . import zeros as _zeros
        if stype is not None:
            grad = _zeros(self.shape, stype=stype)
        else:
            grad = op.zeros_like(self)  # pylint: disable=undefined-variable
        grad_req = _GRAD_REQ_MAP[grad_req]
        check_call(_LIB.MXAutogradMarkVariables(
            1, ctypes.pointer(self.handle),
            ctypes.pointer(mx_uint(grad_req)),
            ctypes.pointer(grad.handle)))