[Bugfix] fix ZeroDivisionError when prefill_tp_size > num_kv_head and fix tp_resharding README (#3437)

### 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>

vllm_ascend/ascend_config.py

@@ -102,7 +102,7 @@ class AscendConfig:
                 )
         self.pd_tp_ratio = 1
         self.pd_head_ratio = 1
-        self.num_head_replica = 0
+        self.num_head_replica = 1
         if vllm_config.kv_transfer_config is not None and not vllm_config.model_config.is_deepseek_mla:
             prefill_tp_size = vllm_config.kv_transfer_config.get_from_extra_config(
                 "prefill", {"tp_size": 1})["tp_size"]
@@ -115,7 +115,7 @@ class AscendConfig:
                     # only support Qwen model now
                     # TODO: use a more robust method to get kv_head_num
                     num_kv_head = vllm_config.model_config.hf_config.num_key_value_heads
-                    self.num_head_replica = prefill_tp_size // num_kv_head
+                    self.num_head_replica = prefill_tp_size // num_kv_head if prefill_tp_size >= num_kv_head else 1
                     prefill_tp_size = min(prefill_tp_size, num_kv_head)
                     decode_tp_size = min(decode_tp_size, num_kv_head)
                     self.pd_head_ratio = prefill_tp_size // decode_tp_size