Fix tsan failure in batch_dataset_op_test.

The error was being caused because we were trying to save invocation_results_` while the function call was in progress. Now we wait for all invocations to finish before saving both `invocation_results_` and `batch_results_`.

Did local A/B testing for tsan.
Before: 12/100 failed
After: All passed
PiperOrigin-RevId: 195141349