### What this PR does / why we need it?
This PR creates and registers `ascend_multi_connector`, which allows the
`mooncake_layerwise_connector` to use the kv_pooling feature.
We unregister the original vllm's `MultiConnector` and replace it with
`AscendMultiConnector` when registering the connectors.
### Does this PR introduce _any_ user-facing change?
No. User can use `MultiConnector` to initialize `AscendMultiConnector`.
### How was this patch tested?
By CI.
- vLLM version: v0.16.0
- vLLM main:
4034c3d32e
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Signed-off-by: nwpu-zxr <zhouxuerong2@huawei.com>
675387f1fd