message: [TF:TRT] Change `AsyncHelper` to call callback function once.
  Macros such as OP_REQUIRES_OK_ASYNC requires AsyncHelper to have this operator, which can be used to invoke the callback function. However, in our case, we only need to invoke the callback function when the object is destructed.
diff: @@ -115,11 +115,11 @@ class AsyncHelper : public core::RefCounted {
  public:
  AsyncHelper(AsyncOpKernel::DoneCallback done) : done_(done) {}
  - ~AsyncHelper() override { this->operator()(); }
  + ~AsyncHelper() override { done_(); }
  - void operator()() {
  - done_();
  - }
  + // The function call operator is used at error handling. However, the callback
  + // is deferred to destruction.
  + void operator()() {}
  private:
  AsyncOpKernel::DoneCallback done_;
  @@ -502,8 +502,9 @@ void TRTEngineOp::ExecuteNativeSegment(OpKernelContext* ctx,
  allow_soft_placement_, ctx->num_inputs(),
  ctx->num_outputs());
  OP_REQUIRES_OK_ASYNC(ctx, status_or_handle.status(), *helper);
  - native_execution_func_handle_ = status_or_handle.ValueOrDie();
  + native_execution_func_handle_ = *status_or_handle;
  }
  +
  auto lib = ctx->function_library();
  FunctionLibraryRuntime::Options opts;
  opts.rendezvous = ctx->rendezvous();
  