Move token related functions to common (#10942)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

common/token_utils.py

@@ -0,0 +1,76 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+
+import os
+import tiktoken
+
+from common.file_utils import get_project_base_directory
+
+tiktoken_cache_dir = get_project_base_directory()
+os.environ["TIKTOKEN_CACHE_DIR"] = tiktoken_cache_dir
+# encoder = tiktoken.encoding_for_model("gpt-3.5-turbo")
+encoder = tiktoken.get_encoding("cl100k_base")
+
+
+def num_tokens_from_string(string: str) -> int:
+    """Returns the number of tokens in a text string."""
+    try:
+        code_list = encoder.encode(string)
+        return len(code_list)
+    except Exception:
+        return 0
+
+def total_token_count_from_response(resp):
+    if resp is None:
+        return 0
+
+    if hasattr(resp, "usage") and hasattr(resp.usage, "total_tokens"):
+        try:
+            return resp.usage.total_tokens
+        except Exception:
+            pass
+
+    if hasattr(resp, "usage_metadata") and hasattr(resp.usage_metadata, "total_tokens"):
+        try:
+            return resp.usage_metadata.total_tokens
+        except Exception:
+            pass
+
+    if 'usage' in resp and 'total_tokens' in resp['usage']:
+        try:
+            return resp["usage"]["total_tokens"]
+        except Exception:
+            pass
+
+    if 'usage' in resp and 'input_tokens' in resp['usage'] and 'output_tokens' in resp['usage']:
+        try:
+            return resp["usage"]["input_tokens"] + resp["usage"]["output_tokens"]
+        except Exception:
+            pass
+
+    if 'meta' in resp and 'tokens' in resp['meta'] and 'input_tokens' in resp['meta']['tokens'] and 'output_tokens' in resp['meta']['tokens']:
+        try:
+            return resp["meta"]["tokens"]["input_tokens"] + resp["meta"]["tokens"]["output_tokens"]
+        except Exception:
+            pass
+    return 0
+
+
+def truncate(string: str, max_len: int) -> str:
+    """Returns truncated text if the length of text exceed max_len."""
+    return encoder.decode(encoder.encode(string)[:max_len])
+