Temporary solution to remove the unnecessary ar cache during prefill

[zhihaoshan-google]

Description

Temporary solution to remove the unnecessary AR cache during prefill. The change can decrease the HBM usage and speed up the KV Cache transfer for the disaggregated serving.

FIXES: b/382716526

Tests

Manual test.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.