fix gibberish when use flaggems atten (#382)

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### PR Category
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OP
### PR Type
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Bug Fixes
### Description
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fix Gibberish when use flaggems atten
### Related Issues
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### Changes
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-
add forward_includes_kv_cache_update=False By default
### Testing
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### Checklist
- [ ] I have run the existing tests and they pass
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Author: ZenithJoyH

vllm_fl/dispatch/backends/flaggems/impl/attention.py

@@ -50,6 +50,11 @@
 
 class AttentionFLBackend(AttentionBackend):
     accept_output_buffer: bool = True
+    # KV cache update is performed by vLLM via do_kv_cache_update() before
+    # forward(). Without this, vLLM assumes the backend updates the KV cache
+    # inside forward() and never calls do_kv_cache_update, leaving the cache
+    # unwritten (all zeros) and producing garbage output.
+    forward_includes_kv_cache_update: bool = False
     supported_dtypes: ClassVar[list[torch.dtype]] = [torch.float16, torch.bfloat16]
 
     @staticmethod