[Tidy] Use BaseChatModel for type hint

[lingyielia]

Description

Motivation:
when adding optional model families, in current setting, LLM_MODELS value varies based on import result

try:
    from langchain_anthropic import ChatAnthropic
except ImportError:
    ChatAnthropic=None

if ChatAnthropic in not None:
    LLM_MODELS = Union[ChatOpenAI, ChatAnthropic]

Use the parent class of all langchain supported chat models
Then we don't need this lines of code:

# TODO add new wrappers in if new model support is added
LLM_MODELS = Union[ChatOpenAI]

if ChatAnthropic in not None:
    LLM_MODELS = Union[ChatOpenAI, ChatAnthropic]

Screenshot

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