Allow prompts to refer to other prompts in Langsmith Hub

[nikhilshinday]

Checked

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

Feature request

Mustache syntax permits you to refer to additional templates using the > partial operator, as in {{> another-template}} which refers to another-template.mustache in your filesystem.

A similar system could be used to include references to other prompts in langchain; ex: I have a ChatPromptTemplate that refers to a TextTemplate with {{lc> another-template}}

Motivation

Two benefits of this approach.

1. composability/reusability: there are common behavioral patterns that are useful in many different chains; an example is writing style or specific details of tool use.
2. observability becomes clearer: Right now, I "compile" prompts by pulling a pair of prompts and injecting one into the other. I lose provenance for which prompt was pulled from. A concrete example: we often have map/reduce prompts that require us to map over data and a list of content that we want to extract out of that data - essentially, a map/reduce in two dimensions. We have a "main" prompt for the mapping step, into which our data and our desired extraction content are injected, and each of the desired extraction contents are themselves prompts; I'd love to have a "main" prompt that each of the extraction contents can refer to inside the prompt template itself, rather than having fragments of prompts that each need to be injected into the "main" prompt. I envision a pretty cool interface in langsmith that allows you to see the provenance of prompts over tracing runs.

Proposal (If applicable)

No response