[Refactor]3/N Refactor mla_v1.py & extract mla_cp (#4933)

RFC: https://github.com/vllm-project/vllm-ascend/issues/4629
Reason：
The functions related to Cp differ significantly from those of normal
MLA-Attention, but the coupling is quite severe.

Steps：
Isolate PCP and DCP
(1) create a new python file: mla_cp.py
(2) add classes AscendMlaCPImpl and
AscendMlaCPMetadataBuilder，Inheritance AscendMLAImpl and
AscendMLAMetadataBuilder
(3) Remove PCP and DCP-related methods from mla_v1.py to mla_cp.py

vLLM version: v0.12.0

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: wujinyuan1 <wjy9595@qq.com>
Co-authored-by: wujinyuan1 <wjy9595@qq.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

tests/ut/attention/test_mla_v1.py

@@ -912,7 +912,6 @@ class TestAscendMLAImpl(TestBase):
         self.assertIsNotNone(self.impl.kv_a_proj_with_mqa)
         self.assertIsNotNone(self.impl.kv_a_layernorm)
         self.assertEqual(self.impl.num_queries_per_kv, 32)
-        self.assertEqual(self.impl.tp_size, 2)
 
     def test_q_proj_and_k_up_proj(self):
         batch_size = 4