<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cast(self, dtype):
 """Cast data and gradient of this Parameter to a new data type. Parameters dtype : str or numpy.dtype The new data type. """

        self.dtype = dtype
        if self._data is None:
            return
        with autograd.pause():
            self._data = [i.astype(dtype) for i in self._data]
            if self._grad is None:
                return
            self._grad = [i.astype(dtype) for i in self._grad]
            autograd.mark_variables(self._data, self._grad, self.grad_req)