[Misc] Add --attention-backend auto option (#35738)

Signed-off-by: NickLucche <nlucches@redhat.com>

vllm/config/attention.py

@@ -14,7 +14,7 @@ class AttentionConfig:
     """Configuration for attention mechanisms in vLLM."""
 
     backend: AttentionBackendEnum | None = None
-    """Attention backend to use. If None, will be selected automatically."""
+    """Attention backend to use. Use "auto" or None for automatic selection."""
 
     flash_attn_version: Literal[2, 3, 4] | None = None
     """Force vllm to use a specific flash-attention version (2, 3, or 4).
@@ -63,7 +63,13 @@ class AttentionConfig:
     @field_validator("backend", mode="before")
     @classmethod
     def validate_backend_before(cls, value: Any) -> Any:
-        """Enable parsing of the `backend` enum type from string."""
+        """Enable parsing of the `backend` enum type from string.
+
+        The special value "auto" is treated as None, which triggers
+        automatic backend selection.
+        """
         if isinstance(value, str):
+            if value.lower() == "auto":
+                return None
             return AttentionBackendEnum[value.upper()]
         return value