Converts a variable to a tensor.

def _dense_var_to_tensor(self, dtype=None, name=None, as_ref=False):
    """Converts a variable to a tensor."""
    # pylint: disable=protected-access
    if _enclosing_tpu_context() is None:
      if hasattr(self._primary_var, '_dense_var_to_tensor'):
        return self._primary_var._dense_var_to_tensor(dtype, name, as_ref)
      else:
        return ops.convert_to_tensor(self._primary_var)
    # pylint: enable=protected-access
    if dtype is not None and dtype != self.dtype:
      return NotImplemented
    if as_ref:
      return self.handle
    else:
      return self.read_value()