Improve TypeScript types of the pick method by adding type inference

[arnaugomez]

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

I would like to improve the type safety of the pick method of the Runnable class.

The pick method returns a Runnable with no types data:
https://v03.api.js.langchain.com/classes/_langchain_core.runnables.Runnable.html#pick

Example:

const original: Runnable<string, {foo: 1, bar: 2, baz: 3}, {}>
const picked = original.pick(["foo", "bar"]) 
// type of picked is Runnable<any, any, RunnableConfig<Record<string, any>>>  😢 

However, with TypeScript's type inference mechanism, we can infer the types of the input and the output. My proposal is to improve the types to make this possible. The result would be:

const original: Runnable<string, {foo: 1, bar: 2, baz: 3}, {}>
const picked = original.pick(["foo", "bar"]) 
// type of picked is Runnable<string, {foo: 1, bar: 2}, RunnableConfig<Record<string, any>>>  😄 

Motivation

We can leverage TypeScript's type inference mechanism to improve the type safety of our application and avoid having to declare the types of our Runnables explicitly at every step of the chain.

Proposal (If applicable)

I have re-written the type declaration of the pick method so that it infers the return type.

This is the code. Should I submit a PR with this change? I'm new to contributing to Langchain, so I don't know if I should wait for a maintainer to create an issue, before I create the PR.

class Runnable<RunInput = any, RunOutput = any, CallOptions extends RunnableConfig = RunnableConfig> extends Serializable implements RunnableInterface<RunInput, RunOutput, CallOptions> {
  /**
   * Pick keys from the dict output of this runnable. Returns a new runnable.
   */
  pick<TKeys extends keyof RunOutput>(
    keys: TKeys | TKeys[]
  ): Runnable<RunInput, Pick<RunOutput, TKeys>, CallOptions>;
}

[arnaugomez]

I created this TypeScript playground where you can try it out and see the type inference in action.