[t5x] Finish inference in case output file was not written fully
[t5x] Finish inference in case output file was not written fully

When job was preempted when writing the final output file, it did not check if the output file was fully written correctly - instead of checking if `{file_path}.COMPLETED` file was present, it checked if `{file_path}` was there, which could have been not written fully.

PiperOrigin-RevId: 467216025

google-research/t5x