Allow ChatPromptTemplate to accept formatting with AIMessage/UserMessage

[syntaxsdev]

Checked

• I searched existing ideas and did not find a similar one
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• I've clearly described the feature request and motivation for it

Feature request

When creating a ChatPromptTemplate, the only way you can format it is if you use a tuple (agent, message) with the format being in the message. As pictured below from langchain doc:

But you can create the same with:

ChatPromptTemplate.from_messages([
SystemMessage("You are {name}, an AI assistant."),
MessagesPlaceholder("messages")])

but if you try to invoke, format, or use format_prompt on that template object, it doesn't work. It seems like this should be a feature/obvious functionality but was never implemented.

Motivation

Important for standardization, as document does not outline this well.

Proposal (If applicable)

No response