[KV] Change padding logic when head_dim is 64

[kyuyeunk]

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[github-actions]

Description

Start with a short description of what the PR does and how this is a change from
the past.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
• some information about the specific implementation,
• shortcomings of the solution and possible future improvements.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/123456
FIXES: #123456

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[yaochengji]

LGTM!

Do you think we need to place the kv cache related logic in one place? Last time I modified here, #976, but apparently it's not enough.

[kyuyeunk]

LGTM!

Do you think we need to place the kv cache related logic in one place? Last time I modified here, #976, but apparently it's not enough.

Agreed on consolidating these logics into one place. Let's mark it as a future todo for now.