[refactor](UT,PCP,DCP) refactor pcp&dcp patches in UTs (#5505)

### What this PR does / why we need it?
Refactor PCP & DCP patches in UTs: Merge and reuse communication groups
and communication function patches to reduce code duplication.
### Does this PR introduce _any_ user-facing change?
No
- vLLM version: v0.13.0
- vLLM main:
45c1ca1ca1

Signed-off-by: QiuChunshuo <qiuchunshuo@huawei.com>

tests/ut/attention/utils.py

@@ -0,0 +1,64 @@
+from functools import wraps
+from unittest.mock import MagicMock, patch
+
+from vllm.distributed.parallel_state import all_gather_fake
+
+
+def patch_distributed_groups(dcp_size=1,
+                             dcp_rank=0,
+                             pcp_size=1,
+                             pcp_rank=0,
+                             needs_mocks=True):
+    """
+    Decorator to patch common distributed group mocks with configuration
+    
+    Args:
+        dcp_size: DCP world size (default: 1)
+        dcp_rank: DCP rank (default: 0)
+        pcp_size: PCP world size (default: 1)
+        pcp_rank: PCP rank (default: 0)
+        needs_mocks: Whether to pass mock objects as the first arguments 
+             after 'self' to the decorated function. 
+             If True, the decorated function receives: 
+                 func(self, mock_all_to_all_single, mock_dcp, mock_pcp, *args, **kwargs)
+             If False, mocks are not passed and function receives: 
+                 func(self, *args, **kwargs)
+             (default: True)
+    """
+
+    def decorator(func):
+
+        @wraps(func)
+        @patch('torch.distributed.all_to_all_single')
+        @patch('vllm.distributed.parallel_state._PCP')
+        @patch('vllm.distributed.parallel_state._DCP')
+        def wrapper(self, mock_dcp, mock_pcp, mock_all_to_all_single, *args,
+                    **kwargs):
+            mock_dcp.rank_in_group = dcp_rank
+            mock_dcp.world_size = dcp_size
+            mock_dcp.device_group = MagicMock()
+
+            mock_dcp.all_gather = MagicMock()
+            mock_dcp.all_gather.side_effect = lambda input_, dim: all_gather_fake(
+                input_, dim, mock_dcp.world_size, "mock_dcp_group")
+
+            mock_pcp.rank_in_group = pcp_rank
+            mock_pcp.world_size = pcp_size
+            mock_pcp.device_group = MagicMock()
+
+            mock_pcp.all_gather = MagicMock()
+            mock_pcp.all_gather.side_effect = lambda input_, dim: all_gather_fake(
+                input_, dim, mock_pcp.world_size, "mock_pcp_group")
+
+            mock_all_to_all_single.side_effect = lambda output, input, *a, **kw: output.copy_(
+                input)
+
+            if needs_mocks:
+                return func(self, mock_all_to_all_single, mock_dcp, mock_pcp,
+                            *args, **kwargs)
+            else:
+                return func(self, *args, **kwargs)
+
+        return wrapper
+
+    return decorator