variable ops, thus allowing them to run in parallel across iterations.

    Before:

    ```mlir
    !tf_res = type tensor<!tf_type.resource<tensor<f32>>>
    func @while_body(%arg0: !tf_res, %arg1: !tf_res, %arg2: tensor<f32>, %arg3: tensor<f32>) -> (!tf_res, !tf_res, tensor<f32>, tensor<f32>) {
      %graph:4 = tf_executor.graph {