### What this PR does / why we need it?
> Extracted from PR #5513
Based on the Sharded-CP feature PR:#4702;
RFC:https://github.com/vllm-project/vllm/issues/30055
### All-gather KV Cache for Communication Overlap:
- This PR adjusts the calculation order in the SFA.
- split `index_select` into `indexer_select_pre_process` and
`indexer_select_post_process`.
- Combine `nope`, `rope` and `index-k` into a tensor to perform
asynchronous all-gather.
### benchmark:
input=40k && num_batch_token=20k
- before:
```
Mean TTFT (ms): 2614.52
Median TTFT (ms): 3148.03
P50 TTFT (ms): 3148.03
P90 TTFT (ms): 3163.48
P99 TTFT (ms): 3170.20
```
- after:
```
Mean TTFT (ms): 2529.92
Median TTFT (ms): 3051.69
P50 TTFT (ms): 3051.69
P90 TTFT (ms): 3067.31
P99 TTFT (ms): 3072.15
```
### Does this PR introduce _any_ user-facing change?
None
### How was this patch tested?
- vLLM version: v0.13.0
- vLLM main:
2f4e6548ef
---------
Signed-off-by: zzhx1 <zzh_201018@outlook.com>
db12c1e2c8