raise atol for MT5OnnxConfig
# What does this PR do?

MT5 is newly added to ONNX tests, but currently failed with
```
AssertionError: mt5, seq2seq-lm -> Outputs values doesn't match between reference model and ONNX exported model: Got max absolute difference of: 0.000148773193359375

AssertionError: mt5, seq2seq-lm-with-past -> Outputs values doesn't match between reference model and ONNX exported model: Got max absolute difference of: 0.00020599365234375
```

[failed job run](https://github.com/huggingface/transformers/runs/7718274393?check_suite_focus=true)
_The documentation is not available anymore as the PR was closed or merged._<|||||>I need to check the `# Copied` issue.<|||||>Hi, @ydshieh. Does this PR fix the two failing tests?

If not, I can work on it too.<|||||>The test is still running. I think it should be fine :-) but will let you know once the test run is finished. Thanks.