[FrontEnd] Make merge_async_iterators is_cancelled arg optional (v…

…llm-project#7282)

vllm/utils.py

@@ -405,29 +405,30 @@ async def iterate_with_cancellation(
 
 async def merge_async_iterators(
     *iterators: AsyncGenerator[T, None],
-    is_cancelled: Callable[[], Awaitable[bool]],
+    is_cancelled: Optional[Callable[[], Awaitable[bool]]] = None,
 ) -> AsyncGenerator[Tuple[int, T], None]:
     """Merge multiple asynchronous iterators into a single iterator.
 
     This method handle the case where some iterators finish before others.
     When it yields, it yields a tuple (i, item) where i is the index of the
     iterator that yields the item.
 
-    It also polls the provided function at least once per second to check
-    for client cancellation.
+    It also optionally polls a provided function at least once per second
+    to check for client cancellation.
     """
 
     # Can use anext() in python >= 3.10
     awaits = {
         ensure_future(pair[1].__anext__()): pair
         for pair in enumerate(iterators)
     }
+    timeout = None if is_cancelled is None else 1
     try:
         while awaits:
             done, pending = await asyncio.wait(awaits.keys(),
                                                return_when=FIRST_COMPLETED,
-                                               timeout=1)
-            if await is_cancelled():
+                                               timeout=timeout)
+            if is_cancelled is not None and await is_cancelled():
                 raise asyncio.CancelledError("client cancelled")
             for d in done:
                 pair = awaits.pop(d)