Fix Mixtral Parity test to keep it consistent with Transformers. (#20210)
### Description
I recently opened a PR in hf transformers repo to fix an issue on the
indexing part.
https://github.com/huggingface/transformers/issues/29857
onnx exporter was failing because of the tolist() conversion so we had
to remove it.
I found out that the code was also a part of our codebase so this PR is
to keep the code consistent.