fix(doc): document -1 sentinel in ForwardInput.write_tuple

[MisakaVan]

Summary

• The inline comment on ForwardInput.write_tuple in scheduler.py documented the sentinel value as 0, but _make_write_tuple actually uses -1 for finished requests that cannot decode.

Reference

_make_write_tuple function:

    write_list = [(req.device_len if req.can_decode else -1) for req in batch.reqs]