Add in-code documentation for LangChain Runnables

[eyurtsev]

Privileged issue

• I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here.

Issue Content

We would love to get help to add in code documentation to LangChain core to better document LCEL primitives:

Here is an example of a documented runnable:
https://api.python.langchain.com/en/latest/runnables/langchain_core.runnables.base.RunnableLambda.html#langchain_core.runnables.base.RunnableLambda

Here is an example of an undocumented runnable (currently many are undocumented): https://api.python.langchain.com/en/latest/runnables/langchain_core.runnables.passthrough.RunnablePick.html

Acceptance Criteria:

• PR should be as minimal as possible (don't try to document unrelated runnables please!) Keep PR size small to get things merged quicker and avoid merge conflicts.
• Document the class doc-string:
  • include an overview about what the runnable does
  • include a ...code-block: python that shows a self-contained example of how to use the runnable.
  • the self contained example should include all relevant imports so it can be copy pasted AS is and run

How do i figure out what the runnable does?

• All runnables have unit tests that show how the runnable can be used! You can locate the unit tests and use them as reference.

Some especially import runnables (note that some of these are base abstractions)

• langchain/libs/core/langchain_core/runnables/passthrough.py

  Line 315 in bc6249c

  class RunnableAssign(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):

• langchain/libs/core/langchain_core/runnables/passthrough.py

  Line 577 in bc6249c

  class RunnablePick(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):

• langchain/libs/core/langchain_core/runnables/configurable.py

  Line 44 in bc6249c

  class DynamicRunnable(RunnableSerializable[Input, Output]):

• langchain/libs/core/langchain_core/runnables/configurable.py

  Line 222 in bc6249c

  class RunnableConfigurableFields(DynamicRunnable[Input, Output]):

• Context:

  langchain/libs/core/langchain_core/beta/runnables/context.py

  Line 309 in bc6249c

  class Context:

[devesh-2002]

May I take this issue?

[eyurtsev]

Yes! Anyone can sign up. Claim any of the runnables and post it here , so other folks can claim other runnables

[devesh-2002]

I would like to work upon RunnablePick

[aanaseer]

I'll take on documenting runnables.passthrough.RunnableAssign.

[aaronjimv]

Hello, I would like to work upon class Context

[spike-spiegel-21]

I have added docs for RunnableConfigurableFields. Please check my PR @eyurtsev
#19849

[eyurtsev]

Thanks everyone for your help!