propagate RetrievalQA chain callbacks through its own LLMChain and StuffDocumentsChain (#7853)

This is another case, similar to #5572 and #7565 where the callbacks are
getting dropped during construction of the chains.

tagging @hwchase17 and @agola11 for callbacks propagation

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libs/langchain/langchain/chains/retrieval_qa/base.py

@@ -11,6 +11,7 @@ from pydantic import Extra, Field, root_validator
 from langchain.callbacks.manager import (
     AsyncCallbackManagerForChainRun,
     CallbackManagerForChainRun,
+    Callbacks,
 )
 from langchain.chains.base import Chain
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
@@ -65,11 +66,12 @@ class BaseRetrievalQA(Chain):
         cls,
         llm: BaseLanguageModel,
         prompt: Optional[PromptTemplate] = None,
+        callbacks: Callbacks = None,
         **kwargs: Any,
     ) -> BaseRetrievalQA:
         """Initialize from LLM."""
         _prompt = prompt or PROMPT_SELECTOR.get_prompt(llm)
-        llm_chain = LLMChain(llm=llm, prompt=_prompt)
+        llm_chain = LLMChain(llm=llm, prompt=_prompt, callbacks=callbacks)
         document_prompt = PromptTemplate(
             input_variables=["page_content"], template="Context:\n{page_content}"
         )
@@ -77,9 +79,14 @@ class BaseRetrievalQA(Chain):
             llm_chain=llm_chain,
             document_variable_name="context",
             document_prompt=document_prompt,
+            callbacks=callbacks,
         )
 
-        return cls(combine_documents_chain=combine_documents_chain, **kwargs)
+        return cls(
+            combine_documents_chain=combine_documents_chain,
+            callbacks=callbacks,
+            **kwargs,
+        )
 
     @classmethod
     def from_chain_type(