Input variables is ignored even though passed in when MessagesPlaceholder.optional=True

Checked other resources

[x] I added a very descriptive title to this issue.
[X] I searched the LangChain documentation with the integrated search.
[X] I used the GitHub search to find a similar question and didn't find it.
[X] I am sure that this is a bug in LangChain rather than my code.
[X] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

Example Code



memory_key = "chat_context"

prompt = ChatPromptTemplate(
    messages=[
        MessagesPlaceholder(variable_name=memory_key, optional=True),
        HumanMessagePromptTemplate.from_template("{question}")
    ]
)

memory = ConversationBufferMemory(memory_key=memory_key, return_messages=True)
conversation = LLMChain(
    llm=llm,
    verbose=True,
    memory=memory,
    prompt=prompt
)

user_input = input("Me: ")
while user_input != "exit":
    print(conversation.invoke({"question": user_input}))
    user_input = input("Me: ")

### Error Message and Stack Trace (if applicable)

![1](https://github.com/langchain-ai/langchain/assets/12031633/419f2b04-a259-4cec-b66a-2d9ce2d5e112)
![2](https://github.com/langchain-ai/langchain/assets/12031633/462381c0-83dc-4ec2-9d79-5662c3c7d292)

### Description

as the title described, when MessagesPlaceholder.optional=True, the input variables will be ignored, even though I passed the arguments in. **I suppose this to be a bug, because it makes no sense when init a MessagesPlaceholder and optional set True.**

or **if this is by design, could you pls share some original design intentions or use cases.**

![3](https://github.com/langchain-ai/langchain/assets/12031633/893cf930-7cdb-40d9-8341-6fa043ce90ed)

### System Info

Name: langchain-core
Version: 0.1.45
Summary: Building applications with LLMs through composability
Home-page: https://github.com/langchain-ai/langchain
Author: 
Author-email: 
License: MIT
Location: /Users/linbo.yuan/Library/Python/3.9/lib/python/site-packages
Requires: PyYAML, tenacity, pydantic, langsmith, jsonpatch, packaging
Required-by: langchain, langchain-text-splitters, langchain-openai, langchain-community

langchain-ai / langchain

Input variables is ignored even though passed in when MessagesPlaceholder.optional=True #21425

Checked other resources

Example Code