style: fix typo and format code (#2618)

### What problem does this PR solve?

- Fix typo
- Remove unused import
- Format code

### Type of change

- [x] Other (please describe): typo and format

rag/utils/__init__.py

@@ -78,11 +78,9 @@ encoder = tiktoken.encoding_for_model("gpt-3.5-turbo")
 def num_tokens_from_string(string: str) -> int:
     """Returns the number of tokens in a text string."""
     try:
-        num_tokens = len(encoder.encode(string))
-        return num_tokens
-    except Exception as e:
-        pass
-    return 0
+        return len(encoder.encode(string))
+    except Exception:
+        return 0
 
 
 def truncate(string: str, max_len: int) -> str: