Writes the number of zeroes in the gradients to Tensorboard.

def _write_num_zeros(self)->None:
        "Writes the number of zeroes in the gradients to Tensorboard."
        gradient_nps = [to_np(x.data) for x in self.gradients]
        num_zeros = sum((np.asarray(x) == 0.0).sum() for x in gradient_nps)
        self._add_gradient_scalar('num_zeros', scalar_value=num_zeros)