message: [TF:XLA] Don't use pow for cubing
  Pow has all sorts of special cases that it must handle which are not
  interesting for use cases which simply cube their input.
diff: @@ -161,7 +161,7 @@ XLA_MAKE_BINARY(RealDiv, xla::Div(lhs, rhs, extend_dimensions));
  XLA_MAKE_BINARY(ReciprocalGrad, xla::Neg(xla::Mul(rhs, xla::Mul(lhs, lhs))));
  XLA_MAKE_BINARY(
  RsqrtGrad,
  - xla::Mul(xla::Pow(lhs, XlaHelpers::IntegerLiteral(b, input_type(0), 3)),
  + xla::Mul((lhs * lhs) * lhs,
  xla::Div(rhs, XlaHelpers::IntegerLiteral(b, input_type(0), -2)),
  extend_dimensions));
  XLA_MAKE_BINARY(
  