Param 'rephrase_question' not skipping question_generator invocation in BaseConversationalRetrievalChain

[dvp-tran]

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Feature request

Slight alteration of source code to be able to use rephrase_question=False to skip question condensor/rephraser invocation and just pass input question to retriever.

Motivation

rephrase_question exisiting in current base class but not skipping the question rephraser invocation destroy the purpose of the parameter it self. Avoiding the invocation can also save cost/compute by skipping the invocation

Proposal (If applicable)

For lines 144 & 196 of libs/langchain/langchain/chains/conversational_retrieval/base.py

if chat_history_str and rephrase_question:
            callbacks = _run_manager.get_child()
            new_question = self.question_generator.run(
                question=question, chat_history=chat_history_str, callbacks=callbacks
            )