[Speculative Decoding] Add speculators config support (#21345)

vllm/engine/arg_utils.py

@@ -978,8 +978,28 @@ class EngineArgs:
         provided as a JSON string input via CLI arguments or directly as a
         dictionary from the engine.
         """
+
+        from vllm.transformers_utils.config import get_config
+        from vllm.transformers_utils.configs.speculators.base import (
+            SpeculatorsConfig)
+
         if self.speculative_config is None:
-            return None
+            hf_config = get_config(self.hf_config_path or self.model,
+                                   self.trust_remote_code, self.revision,
+                                   self.code_revision, self.config_format)
+
+            # if loading a SpeculatorsConfig, load the specualtive_config
+            # details from the config directly
+            # no user input required / expected
+            if isinstance(hf_config, SpeculatorsConfig):
+                # We create one since we dont create one
+                self.speculative_config = {}
+                self.speculative_config[
+                    "num_speculative_tokens"] = hf_config.num_lookahead_tokens
+                self.speculative_config["model"] = self.model
+                self.speculative_config["method"] = hf_config.method
+            else:
+                return None
 
         # Note(Shangming): These parameters are not obtained from the cli arg
         # '--speculative-config' and must be passed in when creating the engine