message: Set collective_executor in function CallOp from OpKernelContext.
  Without this change, CollectiveOps in functions cannot execute.
diff: @@ -310,6 +310,7 @@ class CallOp : public AsyncOpKernel {
  opts.step_container = ctx->step_container();
  opts.stats_collector = ctx->stats_collector();
  opts.runner = ctx->runner();
  + opts.collective_executor = ctx->collective_executor();
  std::vector<Tensor> args;
  args.reserve(ctx->num_inputs());
  for (int i = 0; i < ctx->num_inputs(); ++i) {
  