[LLMLingua2] Fix wrong key storage during filtering

[noiji]

What does this PR do?

This PR modifies the code to store retrieval in res_pt instead of kept, addressing potential issues that could arise during data verification.

This change does not affect the training results directly, but resolves potential data verification issues by properly associating retrieval with kept.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

@iofu728 @QianhuiWu