<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def as_list(self):
 """Returns a list of integers or `None` for each dimension. Returns: A list of integers or `None` for each dimension. Raises: ValueError: If `self` is an unknown shape with an unknown rank. """

        if self._dims is None:
            raise ValueError("as_list() is not defined on an unknown TensorShape.")
        return [dim.value for dim in self._dims]