Add allowed and disallowed special arguments to BaseOpenAI (#3012)

## Background
This PR fixes this error when there are special tokens when querying the
chain:
```
Encountered text corresponding to disallowed special token '<|endofprompt|>'.
If you want this text to be encoded as a special token, pass it to `allowed_special`, e.g. `allowed_special={'<|endofprompt|>', ...}`.
If you want this text to be encoded as normal text, disable the check for this token by passing `disallowed_special=(enc.special_tokens_set - {'<|endofprompt|>'})`.
To disable this check for all special tokens, pass `disallowed_special=()`.
```

Refer to the code snippet below, it breaks in the chain line.
```
        chain = ConversationalRetrievalChain.from_llm(
            ChatOpenAI(openai_api_key=OPENAI_API_KEY),
            retriever=vectorstore.as_retriever(),
            qa_prompt=prompt,
            condense_question_prompt=condense_prompt,
        )
        answer = chain({"question": f"{question}"})
```
However `ChatOpenAI` class is not accepting `allowed_special` and
`disallowed_special` at the moment so they cannot be passed to the
`encode()` in `get_num_tokens` method to avoid the errors.


## Change
- Add `allowed_special` and `disallowed_special` attributes to
`BaseOpenAI` class.
- Pass in `allowed_special` and `disallowed_special` as arguments of
`encode()` in tiktoken.

---------

Co-authored-by: samcarmen <“carmen.samkahman@gmail.com”>

langchain/llms/openai.py

@@ -5,11 +5,14 @@ import logging
 import sys
 import warnings
 from typing import (
+    AbstractSet,
     Any,
     Callable,
+    Collection,
     Dict,
     Generator,
     List,
+    Literal,
     Mapping,
     Optional,
     Set,
@@ -150,6 +153,10 @@ class BaseOpenAI(BaseLLM):
     """Maximum number of retries to make when generating."""
     streaming: bool = False
     """Whether to stream the results or not."""
+    allowed_special: Union[Literal["all"], AbstractSet[str]] = set()
+    """Set of special tokens that are allowed。"""
+    disallowed_special: Union[Literal["all"], Collection[str]] = "all"
+    """Set of special tokens that are not allowed。"""
 
     def __new__(cls, **data: Any) -> Union[OpenAIChat, BaseOpenAI]:  # type: ignore
         """Initialize the OpenAI object."""
@@ -449,7 +456,11 @@ class BaseOpenAI(BaseLLM):
 
         enc = tiktoken.encoding_for_model(self.model_name)
 
-        tokenized_text = enc.encode(text)
+        tokenized_text = enc.encode(
+            text,
+            allowed_special=self.allowed_special,
+            disallowed_special=self.disallowed_special,
+        )
 
         # calculate the number of tokens in the encoded text
         return len(tokenized_text)
@@ -602,6 +613,10 @@ class OpenAIChat(BaseLLM):
     """Series of messages for Chat input."""
     streaming: bool = False
     """Whether to stream the results or not."""
+    allowed_special: Union[Literal["all"], AbstractSet[str]] = set()
+    """Set of special tokens that are allowed。"""
+    disallowed_special: Union[Literal["all"], Collection[str]] = "all"
+    """Set of special tokens that are not allowed。"""
 
     class Config:
         """Configuration for this pydantic object."""
@@ -785,7 +800,11 @@ class OpenAIChat(BaseLLM):
         enc = tiktoken.encoding_for_model("gpt-3.5-turbo")
 
         # encode the text using the GPT-3.5-Turbo encoder
-        tokenized_text = enc.encode(text)
+        tokenized_text = enc.encode(
+            text,
+            allowed_special=self.allowed_special,
+            disallowed_special=self.disallowed_special,
+        )
 
         # calculate the number of tokens in the encoded text
         return len(tokenized_text)