Change type annotations from LLMChain to Chain in MultiPromptChain (#11082)

- **Description:** The types of 'destination_chains' and 'default_chain'
in 'MultiPromptChain' were changed from 'LLMChain' to 'Chain'. and
removed variables declared overlapping with the parent class
- **Issue:** When a class that inherits only Chain and not LLMChain,
such as 'SequentialChain' or 'RetrievalQA', is entered in
'destination_chains' and 'default_chain', a pydantic validation error is
raised.
-  -  codes
```
retrieval_chain = ConversationalRetrievalChain(
        retriever=doc_retriever,
        combine_docs_chain=combine_docs_chain,
        question_generator=question_gen_chain,
    )
    
    destination_chains = {
        'retrieval': retrieval_chain,
    }
    
    main_chain = MultiPromptChain(
        router_chain=router_chain,
        destination_chains=destination_chains,
        default_chain=default_chain,
        verbose=True,
    )
```

✅ `make format`, `make lint` and `make test`

libs/langchain/langchain/chains/router/multi_prompt.py

@@ -1,11 +1,12 @@
 """Use a single chain to route an input to one of multiple llm chains."""
 from __future__ import annotations
 
-from typing import Any, Dict, List, Mapping, Optional
+from typing import Any, Dict, List, Optional
 
 from langchain.chains import ConversationChain
+from langchain.chains.base import Chain
 from langchain.chains.llm import LLMChain
-from langchain.chains.router.base import MultiRouteChain, RouterChain
+from langchain.chains.router.base import MultiRouteChain
 from langchain.chains.router.llm_router import LLMRouterChain, RouterOutputParser
 from langchain.chains.router.multi_prompt_prompt import MULTI_PROMPT_ROUTER_TEMPLATE
 from langchain.prompts import PromptTemplate
@@ -15,13 +16,6 @@ from langchain.schema.language_model import BaseLanguageModel
 class MultiPromptChain(MultiRouteChain):
     """A multi-route chain that uses an LLM router chain to choose amongst prompts."""
 
-    router_chain: RouterChain
-    """Chain for deciding a destination chain and the input to it."""
-    destination_chains: Mapping[str, LLMChain]
-    """Map of name to candidate chains that inputs can be routed to."""
-    default_chain: LLMChain
-    """Default chain to use when router doesn't map input to one of the destinations."""
-
     @property
     def output_keys(self) -> List[str]:
         return ["text"]
@@ -31,7 +25,7 @@ class MultiPromptChain(MultiRouteChain):
         cls,
         llm: BaseLanguageModel,
         prompt_infos: List[Dict[str, str]],
-        default_chain: Optional[LLMChain] = None,
+        default_chain: Optional[Chain] = None,
         **kwargs: Any,
     ) -> MultiPromptChain:
         """Convenience constructor for instantiating from destination prompts."""