[Doc] Update doc of custom logit processor (#3021)

Signed-off-by: Hongpeng Guo <hpguo@anyscale.com>

docs/references/sampling_params.md

@@ -32,6 +32,20 @@ class GenerateReqInput:
     return_text_in_logprobs: bool = False
     # Whether to stream output.
     stream: bool = False
+    # Whether to log metrics for this request (e.g. health_generate calls do not log metrics)
+    log_metrics: bool = True
+
+    # The modalities of the image data [image, multi-images, video]
+    modalities: Optional[List[str]] = None
+    # LoRA related
+    lora_path: Optional[Union[List[Optional[str]], Optional[str]]] = None
+
+    # Session info for continual prompting
+    session_params: Optional[Union[List[Dict], Dict]] = None
+    # Custom logit processor for advanced sampling control. Must be a serialized instance
+    # of `CustomLogitProcessor` in python/sglang/srt/sampling/custom_logit_processor.py
+    # Use the processor's `to_str()` method to generate the serialized string.
+    custom_logit_processor: Optional[Union[List[Optional[str]], str]] = None
 ```
 
 The `sampling_params` follows this format
@@ -90,6 +104,14 @@ repetition_penalty: float = 1.0,
 # difficult to infer the correct token ID by given `stop` strings.
 # Must be 0 <= value < max_new_tokens. Setting to 0 (default) will disable this penalty.
 min_new_tokens: int = 0,
+
+
+## Custom Parameters for Custom Logit Processor.
+# A dictionary of custom parameters for the custom logit processor.
+# The custom logit processor takes a list of dictionaries as input, where each
+# dictionary is the custom parameters for one token in a batch of the input.
+# See also python/sglang/srt/sampling/custom_logit_processor.py
+custom_params: Optional[Dict[str, Any]] = None,
 ```
 
 ## Examples
@@ -253,3 +275,49 @@ response = requests.post(
 )
 print(response.json())
 ```
+### Custom Logit Processor
+Launch a server with `--enable-custom-logit-processor` flag on.
+```
+python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3-8B-Instruct --port 30000 --enable-custom-logit-processor
+```
+
+Define a custom logit processor that will always sample a specific token id.
+```python
+from sglang.srt.sampling.custom_logit_processor import CustomLogitProcessor
+
+class DeterministicLogitProcessor(CustomLogitProcessor):
+    """A dummy logit processor that changes the logits to always
+    sample the given token id.
+    """
+
+    def __call__(self, logits, custom_param_list):
+        # Check that the number of logits matches the number of custom parameters
+        assert logits.shape[0] == len(custom_param_list)
+        key = "token_id"
+
+        for i, param_dict in enumerate(custom_param_list):
+            # Mask all other tokens
+            logits[i, :] = -float("inf")
+            # Assign highest probability to the specified token
+            logits[i, param_dict[key]] = 0.0
+        return logits
+```
+
+Send a request
+```python
+import requests
+
+response = requests.post(
+    "http://localhost:30000/generate",
+    json={
+        "text": "The capital of France is",
+        "custom_logit_processor": DeterministicLogitProcessor().to_str(),
+        "sampling_params": {
+            "temperature": 0.0,
+            "max_new_tokens": 32,
+            "custom_params": {"token_id": 5},
+        },
+    },
+)
+print(response.json())
+```

python/sglang/srt/managers/io_struct.py

@@ -69,8 +69,10 @@ class GenerateReqInput:
 
     # Session info for continual prompting
     session_params: Optional[Union[List[Dict], Dict]] = None
-    # Custom logit processor (serialized function)
-    custom_logit_processor: Optional[Union[List[Optional[str]], Optional[str]]] = None
+    # Custom logit processor for advanced sampling control. Must be a serialized instance
+    # of `CustomLogitProcessor` in python/sglang/srt/sampling/custom_logit_processor.py
+    # Use the processor's `to_str()` method to generate the serialized string.
+    custom_logit_processor: Optional[Union[List[Optional[str]], str]] = None
 
     def normalize_batch_and_arguments(self):
         if (
@@ -248,8 +250,9 @@ class TokenizedGenerateReqInput:
     # Session info for continual prompting
     session_params: Optional[SessionParams] = None
 
-    # Custom logit processor (serialized function)
-    # TODO (hpguo): Add an example and update doc string here
+    # Custom logit processor for advanced sampling control. Must be a serialized instance
+    # of `CustomLogitProcessor` in python/sglang/srt/sampling/custom_logit_processor.py
+    # Use the processor's `to_str()` method to generate the serialized string.
     custom_logit_processor: Optional[str] = None