Improve documentation of ModelConfig.try_get_generation_config to prevent future confusion (#21526)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config.py

@@ -1575,7 +1575,18 @@ class ModelConfig:
         return self.multimodal_config
 
     def try_get_generation_config(self) -> dict[str, Any]:
-        if self.generation_config in ("auto", "vllm"):
+        """
+        This method attempts to retrieve the non-default values of the
+        generation config for this model.
+        
+        The generation config can contain information about special tokens, as
+        well as sampling parameters. Which is why this method exists separately
+        to `get_diff_sampling_param`.
+
+        Returns:
+            A dictionary containing the non-default generation config.
+        """
+        if self.generation_config in {"auto", "vllm"}:
             config = try_get_generation_config(
                 self.hf_config_path or self.model,
                 trust_remote_code=self.trust_remote_code,
@@ -1594,13 +1605,18 @@ class ModelConfig:
 
     def get_diff_sampling_param(self) -> dict[str, Any]:
         """
-        This method returns a dictionary containing the parameters
+        This method returns a dictionary containing the non-default sampling
-        that differ from the default sampling parameters. If
+        parameters with `override_generation_config` applied.
-        `generation_config` is `"vllm"`, an empty dictionary is returned.
+
+        The default sampling parameters are:
+
+        - vLLM's neutral defaults if `self.generation_config="vllm"`
+        - the model's defaults if `self.generation_config="auto"`
+        - as defined in `generation_config.json` if
+            `self.generation_config="path/to/generation_config/dir"`
 
         Returns:
-            dict[str, Any]: A dictionary with the differing sampling
+            A dictionary containing the non-default sampling parameters.
-            parameters, if `generation_config` is `"vllm"` an empty dictionary.
         """
         if self.generation_config == "vllm":
             config = {}