### What this PR does / why we need it? Fix ZeroDivisionError when prefill_tp_size > num_kv_head, in this situation, num_head_replica can be 0 and used to divide another value, this PR restricts the minimum value of a to be 1. And this PR fix tp_resharding README. ### Does this PR introduce _any_ user-facing change? No. ### How was this patch tested? By CI. - vLLM version: v0.11.0rc3 - vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0 --------- Signed-off-by: liziyu <liziyu16@huawei.com> Signed-off-by: nwpu-zxr <zhouxuerong2@huawei.com> Co-authored-by: liziyu <liziyu16@huawei.com>