server: fix nltk download

server/reflector/llm/base.py

@@ -6,17 +6,17 @@ from typing import TypeVar
 
 import nltk
 from prometheus_client import Counter, Histogram
-from transformers import GenerationConfig
-
 from reflector.llm.llm_params import TaskParams
 from reflector.logger import logger as reflector_logger
 from reflector.settings import settings
 from reflector.utils.retry import retry
+from transformers import GenerationConfig
 
 T = TypeVar("T", bound="LLM")
 
 
 class LLM:
+    _nltk_downloaded = False
     _registry = {}
     m_generate = Histogram(
         "llm_generate",
@@ -39,18 +39,17 @@ class LLM:
         ["backend"],
     )
 
-    def __enter__(self):
-        self.ensure_nltk()
-
     @classmethod
     def ensure_nltk(cls):
         """
         Make sure NLTK package is installed. Searches in the cache and
         downloads only if needed.
         """
-        nltk.download("punkt")
-        # For POS tagging
-        nltk.download("averaged_perceptron_tagger")
+        if not cls._nltk_downloaded:
+            nltk.download("punkt")
+            # For POS tagging
+            nltk.download("averaged_perceptron_tagger")
+            cls._nltk_downloaded = True
 
     @classmethod
     def register(cls, name, klass):
@@ -70,6 +69,7 @@ class LLM:
         if name not in cls._registry:
             module_name = f"reflector.llm.llm_{name}"
             importlib.import_module(module_name)
+        cls.ensure_nltk()
         return cls._registry[name](model_name)
 
     def get_model_name(self) -> str: