Can't use RunnablePassthrough
Labels: name: auto:bug, name: area: models, name: integration: aws

### System Info

Im trying to implement this in sagemaker with bedrock claude v2
https://github.com/langchain-ai/langchain/blob/master/templates/rag-aws-bedrock/rag_aws_bedrock/chain.py

Here is my code
```
`import os

from langchain.embeddings import BedrockEmbeddings
from langchain.llms.bedrock import Bedrock
from langchain.prompts import ChatPromptTemplate
from langchain.vectorstores import FAISS
from langchain_core.output_parsers import StrOutputParser
from langchain_core.pydantic_v1 import BaseModel
from langchain_core.runnables import RunnableParallel, RunnablePassthrough

region = "us-east-1"
model_id="anthropic.claude-v2"
# Set LLM and embeddings
model = Bedrock(
    model_id=model_id,
    region_name=region,
    model_kwargs={"max_tokens_to_sample": 200},
)
bedrock_embeddings = BedrockEmbeddings(model_id="amazon.titan-embed-text-v1")

# Add to vectorDB
vectorstore = FAISS.from_texts(
    ["harrison worked at kensho"], embedding=bedrock_embeddings
)
retriever = vectorstore.as_retriever()

# Get retriever from vectorstore
retriever = vectorstore.as_retriever()

# RAG prompt
template = """Answer the question based only on the following context:
{context}
Question: {question}
"""
prompt = ChatPromptTemplate.from_template(template)


# RAG
chain = (
    RunnableParallel({"context": retriever, "question": RunnablePassthrough()})
    | prompt
    | model
    | StrOutputParser()
)
`

```
got unexpected error:
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[47], line 40
     35 prompt = ChatPromptTemplate.from_template(template)
     38 # RAG
     39 chain = (
---> 40     RunnableParallel({"context": retriever, "question": RunnablePassthrough()})
     41     | prompt
     42     | model
     43     | StrOutputParser()
     44 )

TypeError: Expected a Runnable, callable or dict.Instead got an unsupported type: <class 'langchain.schema.vectorstore.VectorStoreRetriever'>


Name: langchain
Version: 0.0.352

Name: langchain-core
Version: 0.1.2

### Who can help?

_No response_

### Information

- [ ] The official example notebooks/scripts
- [ ] My own modified scripts

### Related Components

- [X] LLMs/Chat Models
- [ ] Embedding Models
- [ ] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [ ] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [X] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction

1. !pip install langchain==0.0.352
2.
```
 import os

from langchain.embeddings import BedrockEmbeddings
from langchain.llms.bedrock import Bedrock
from langchain.prompts import ChatPromptTemplate
from langchain.vectorstores import FAISS
from langchain_core.output_parsers import StrOutputParser
from langchain_core.pydantic_v1 import BaseModel
from langchain_core.runnables import RunnableParallel, RunnablePassthrough

region = "us-east-1"
model_id="anthropic.claude-v2"
# Set LLM and embeddings
model = Bedrock(
    model_id=model_id,
    region_name=region,
    model_kwargs={"max_tokens_to_sample": 200},
)
bedrock_embeddings = BedrockEmbeddings(model_id="amazon.titan-embed-text-v1")

# Add to vectorDB
vectorstore = FAISS.from_texts(
    ["harrison worked at kensho"], embedding=bedrock_embeddings
)
retriever = vectorstore.as_retriever()

# Get retriever from vectorstore
retriever = vectorstore.as_retriever()

# RAG prompt
template = """Answer the question based only on the following context:
{context}
Question: {question}
"""
prompt = ChatPromptTemplate.from_template(template)


# RAG
chain = (
    RunnableParallel({"context": retriever, "question": RunnablePassthrough()})
    | prompt
    | model
    | StrOutputParser()
)

```

### Expected behavior

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[47], line 40
     35 prompt = ChatPromptTemplate.from_template(template)
     38 # RAG
     39 chain = (
---> 40     RunnableParallel({"context": retriever, "question": RunnablePassthrough()})
     41     | prompt
     42     | model
     43     | StrOutputParser()
     44 )

File /opt/conda/lib/python3.10/site-packages/langchain_core/runnables/base.py:1937, in RunnableParallel.__init__(self, _RunnableParallel__steps, **kwargs)
   1934 merged = {**__steps} if __steps is not None else {}
   1935 merged.update(kwargs)
   1936 super().__init__(
-> 1937     steps={key: coerce_to_runnable(r) for key, r in merged.items()}
   1938 )

File /opt/conda/lib/python3.10/site-packages/langchain_core/runnables/base.py:1937, in <dictcomp>(.0)
   1934 merged = {**__steps} if __steps is not None else {}
   1935 merged.update(kwargs)
   1936 super().__init__(
-> 1937     steps={key: coerce_to_runnable(r) for key, r in merged.items()}
   1938 )

File /opt/conda/lib/python3.10/site-packages/langchain_core/runnables/base.py:3232, in coerce_to_runnable(thing)
   3230     return cast(Runnable[Input, Output], RunnableParallel(thing))
   3231 else:
-> 3232     raise TypeError(
   3233         f"Expected a Runnable, callable or dict."
   3234         f"Instead got an unsupported type: {type(thing)}"
   3235     )

TypeError: Expected a Runnable, callable or dict.Instead got an unsupported type: <