Remove graph_pool as member of VllmBackend and argument to CUDAGraphWrapper

[Copilot]

This PR removes graph_pool as a member variable of VllmBackend and as an argument to CUDAGraphWrapper and its base class AbstractStaticGraphWrapper. Instead, CUDAGraphWrapper now always obtains the graph pool directly from the platform using current_platform.get_global_graph_pool().

Changes Made

1. AbstractStaticGraphWrapper Protocol

• Removed graph_pool: Any parameter from __init__ method signature
• Updated docstring to remove graph_pool documentation

2. CUDAGraphWrapper

• Removed graph_pool: Any = None parameter from constructor
• Simplified initialization to always get graph pool from platform:

  self.graph_pool = current_platform.get_global_graph_pool()

• Removed conditional logic that previously checked if graph_pool was None

3. VllmBackend

• Removed self.graph_pool member variable from __init__
• Removed graph_pool: Any from class type annotations
• Updated PiecewiseCompileInterpreter to not take or use graph_pool parameter
• Updated static graph wrapper instantiation to not pass graph_pool argument

Rationale

This change simplifies the architecture by removing an unnecessary parameter that was being passed around when the graph pool can be obtained directly from the platform. The existing fallback logic in CUDAGraphWrapper already called current_platform.get_global_graph_pool() when graph_pool=None, so this change makes that the default behavior while eliminating the parameter passing overhead.

Backward Compatibility

This is a breaking change for any external code that:

• Instantiates CUDAGraphWrapper with the graph_pool argument
• Accesses the graph_pool member of VllmBackend
• Implements AbstractStaticGraphWrapper with the old signature

However, all internal usage has been updated and existing tests continue to work without modification.

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