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
