[Frontend] Gracefully handle missing chat template and fix CI failure (#7238)

Co-authored-by: Roger Wang <ywang@roblox.com>

vllm/entrypoints/chat_utils.py

@@ -1,8 +1,9 @@
 import codecs
 from dataclasses import dataclass
 from functools import lru_cache
-from typing import (Awaitable, Iterable, List, Optional, Tuple, Union, cast,
-                    final)
+from pathlib import Path
+from typing import (Any, Awaitable, Iterable, List, Optional, Tuple, Union,
+                    cast, final)
 
 # yapf conflicts with isort for this block
 # yapf: disable
@@ -22,6 +23,7 @@ from vllm.config import ModelConfig
 from vllm.logger import init_logger
 from vllm.multimodal import MultiModalDataDict
 from vllm.multimodal.utils import async_get_and_parse_image
+from vllm.transformers_utils.tokenizer import AnyTokenizer
 
 logger = init_logger(__name__)
 
@@ -69,13 +71,17 @@ class ChatMessageParseResult:
     mm_futures: List[Awaitable[MultiModalDataDict]]
 
 
-def load_chat_template(chat_template: Optional[str]) -> Optional[str]:
+def load_chat_template(
+        chat_template: Optional[Union[Path, str]]) -> Optional[str]:
     if chat_template is None:
         return None
     try:
         with open(chat_template, "r") as f:
             resolved_chat_template = f.read()
     except OSError as e:
+        if isinstance(chat_template, Path):
+            raise
+
         JINJA_CHARS = "{}\n"
         if not any(c in chat_template for c in JINJA_CHARS):
             msg = (f"The supplied chat template ({chat_template}) "
@@ -208,3 +214,28 @@ def parse_chat_messages(
         mm_futures.extend(parse_result.mm_futures)
 
     return conversation, mm_futures
+
+
+def apply_chat_template(
+    tokenizer: AnyTokenizer,
+    conversation: List[ConversationMessage],
+    chat_template: Optional[str],
+    *,
+    tokenize: bool = False,  # Different from HF's default
+    **kwargs: Any,
+) -> str:
+    if chat_template is None and tokenizer.chat_template is None:
+        raise ValueError(
+            "As of transformers v4.44, default chat template is no longer "
+            "allowed, so you must provide a chat template if the tokenizer "
+            "does not define one.")
+
+    prompt = tokenizer.apply_chat_template(
+        conversation=conversation,
+        chat_template=chat_template,
+        tokenize=tokenize,
+        **kwargs,
+    )
+    assert isinstance(prompt, str)
+
+    return prompt