misc: fix the req_to_token member change

[hnyls2002]

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Motivation

This may cause some error.

Modification

can_use_mem_size -> free_slots

Checklist

1. Ensure pre-commit pre-commit run --all-files or other linting tools are used to fix potential lint issues.
2. Confirm that modifications are covered by complete unit tests. If not, please add more unit tests for correctness.
3. Modify documentation as needed, such as docstrings or example tutorials.