[misc] add forward context for attention (#9029)

vllm/forward_context.py

@@ -0,0 +1,22 @@
+from contextlib import contextmanager
+from typing import Any
+
+_forward_context: Any = None
+
+
+def get_forward_context() -> Any:
+    """Get the current forward context."""
+    return _forward_context
+
+
+@contextmanager
+def set_forward_context(context: Any):
+    """A context manager that stores the current forward context,
+    can be attention metadata, etc."""
+    global _forward_context
+    prev_context = _forward_context
+    _forward_context = context
+    try:
+        yield
+    finally:
+        _forward_context = prev_context