[Python] Extract common device str parse function in ChatModule (mlc-ai#1074)

This PR lifts the device string parsing (just a few of lines)
to a standalone function, so that on the serving side the serving
can make use of this function as well.

Tested Python API and it does not seem to incur regression.

Author: MasterJH5574

python/mlc_chat/chat_module.py

@@ -548,6 +548,38 @@ def _convert_generation_config_to_json_str(generation_config: Optional[Generatio
     return json.dumps(asdict(generation_config))
 
 
+def _parse_device_str(device: str):
+    """Parse the input device identifier into device name and id.
+
+    Parameters
+    ----------
+    device : str
+        The device identifier to parse.
+        It can be "device_name" (e.g., "cuda") or
+        "device_name:device_id" (e.g., "cuda:1").
+
+    Returns
+    -------
+    device_name : str
+        The name of the device.
+
+    device_id : int
+        The id of the device, or 0 if not specified in the input.
+    """
+    device_err_msg = (
+        f"Invalid device name: {device}. Please enter the device in the form "
+        "'device_name:device_id' or 'device_name', where 'device_name' needs to be "
+        "one of 'cuda', 'metal', 'vulkan', 'rocm', 'opencl', 'auto'."
+    )
+    device_args = device.split(":")
+    if len(device_args) == 1:
+        return device_args[0], 0
+    elif len(device_args) == 2:
+        return device_args[0], int(device_args[1])
+    elif len(device_args) > 2:
+        raise ValueError(device_err_msg)
+
+
 def _detect_local_device(device_id: int = 0):
     """Automatically detect the local device if user does not specify.
 
@@ -647,13 +679,7 @@ def __init__(
         )
 
         # 0. Retrieve device_name and device_id (if any, default 0) from device arg
-        device_args = device.split(":")
-        if len(device_args) == 1:
-            device_name, device_id = device_args[0], 0
-        elif len(device_args) == 2:
-            device_name, device_id = device_args[0], int(device_args[1])
-        elif len(device_args) > 2:
-            raise ValueError(device_err_msg)
+        device_name, device_id = _parse_device_str(device)
 
         # 1. Get self.device
         if device_name == "cuda":