if (!isa<func::FuncOp, IfOp, WhileOp>(contraction->getParentOp())) {
      return rewriter.notifyMatchFailure(
          contraction,
          "fused operation must be nested inside a function, If or While");
    }

    // If the contraction is used in multiple places, fusing it will only create
    // more contraction nodes, which is slower.
    if (!contraction.getResult().hasOneUse())

                     absl::string_view output_subs) {
  // Claim: For the einsum operation z = einsum("{eq_x},{eq_y}->{eq_z}", x, y),
  //   where the equation involves only Tensor contractions, generalized traces
  //   and transposes, the input gradients are given by the vector-jacobian
  //   products (VJPs):
  //
  //     grad_wrt_x = einsum("{eq_y},{eq_z}->{eq_x}", y, grad_wrt_z)