[executorch][cuda] Optimize short-query INT6 matvec kernels (#21504)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: https://github.com/pytorch/executorch/pull/21474 by
@Gasoonjia
^ Please use this as the source of truth for the PR details, comments,
and reviews
ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/gasoonjia/179/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/gasoonjia/179/head
Merge bot PR base:
https://github.com/pytorch/executorch/tree/gh/gasoonjia/178/orig
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/gasoonjia/179/orig
Differential Revision:
[D114032330](https://our.internmc.facebook.com/intern/diff/D114032330/)
@diff-train-skip-merge

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Co-authored-by: gasoonjia <gasoonjia@icloud.com>
Co-authored-by: Gasoonjia <gasoonjia@meta.com>