Fix type annotation for SQLDatabaseToolkit.llm (#3581)

Currently `langchain.agents.agent_toolkits.SQLDatabaseToolkit` has a
field `llm` with type `BaseLLM`. This breaks initialization for some
LLMs. For example, trying to use it with GPT4:
```

from langchain.sql_database import SQLDatabase
from langchain.chat_models import ChatOpenAI
from langchain.agents.agent_toolkits import SQLDatabaseToolkit


db = SQLDatabase.from_uri("some_db_uri")
llm = ChatOpenAI(model_name="gpt-4")
toolkit = SQLDatabaseToolkit(db=db, llm=llm)

# pydantic.error_wrappers.ValidationError: 1 validation error for SQLDatabaseToolkit
# llm
#  Can't instantiate abstract class BaseLLM with abstract methods _agenerate, _generate, _llm_type (type=type_error)
```
Seems like much of the rest of the codebase has switched from BaseLLM to
BaseLanguageModel. This PR makes the change for SQLDatabaseToolkit as
well

langchain/agents/agent_toolkits/sql/toolkit.py

@@ -4,7 +4,7 @@ from typing import List
 from pydantic import Field
 
 from langchain.agents.agent_toolkits.base import BaseToolkit
-from langchain.llms.base import BaseLLM
+from langchain.schema import BaseLanguageModel
 from langchain.sql_database import SQLDatabase
 from langchain.tools import BaseTool
 from langchain.tools.sql_database.tool import (
@@ -19,7 +19,7 @@ class SQLDatabaseToolkit(BaseToolkit):
     """Toolkit for interacting with SQL databases."""
 
     db: SQLDatabase = Field(exclude=True)
-    llm: BaseLLM = Field(exclude=True)
+    llm: BaseLanguageModel = Field(exclude=True)
 
     @property
     def dialect(self) -> str: