[ModelRunner][Refactor] Refactor kv cache tensor initialization logic (#3106)

### What this PR does / why we need it?
Refactor kv cache tensor initialization logic. 
1. Unify the kvcache tensor initialization logic of deepseek and normal
models
2. spilt `initialize_kv_cache_tensors` into `_allocate_kv_cache_tensors`
and `_reshape_kv_cache_tensors`, following gpu modelrunner in vllm

### Does this PR introduce _any_ user-facing change?
N/A

### How was this patch tested?
CI passed with existing test.
1. prefill disaggregation scenario
4. deepseek + aclgraph/eager mode
5. qwen3 next


- vLLM version: v0.11.0
- vLLM main:
vllm-project/vllm@83f478b

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Signed-off-by: MengqingCao <[email protected]>

Author: MengqingCao