Revert "refactor: replace gevent threadpool with ProcessPoolExecutor in GPT2Tokenizer"

This reverts commit 1886bb1.

Author: laipz8200

api/core/model_runtime/model_providers/__base/tokenizers/gpt2_tokenzier.py

@@ -1,13 +1,13 @@
-from concurrent.futures import ProcessPoolExecutor
 from os.path import abspath, dirname, join
 from threading import Lock
 from typing import Any, cast
 
+import gevent.threadpool  # type: ignore
 from transformers import GPT2Tokenizer as TransformerGPT2Tokenizer  # type: ignore
 
 _tokenizer: Any = None
 _lock = Lock()
-_executor = ProcessPoolExecutor(max_workers=1)
+_pool = gevent.threadpool.ThreadPool(1)
 
 
 class GPT2Tokenizer:
@@ -22,8 +22,8 @@ def _get_num_tokens_by_gpt2(text: str) -> int:
 
     @staticmethod
     def get_num_tokens(text: str) -> int:
-        future = _executor.submit(GPT2Tokenizer._get_num_tokens_by_gpt2, text)
-        result = future.result()
+        future = _pool.spawn(GPT2Tokenizer._get_num_tokens_by_gpt2, text)
+        result = future.get(block=True)
         return cast(int, result)
 
     @staticmethod