Improve TypeScript types of the assign 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 assign method of the Runnable class.

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

Example:

const original: Runnable<string, {foo: 1, bar: 2, baz: 3}, {}>
const assigned = original.assign({baz: RunnableLambda.from(() => true)}) 
// type of assigned 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 assigned = original.assign({ baz: RunnableLambda.from(() => true) }) 
// type of assigned is Runnable<string, {foo: 1, bar: 2, baz: boolean}, RunnableConfig<Record<string, any>>>  😄 

Motivation

We can leverage TypeScript's type inference mechanism to improve the type safety of our application. This way, we can have autocompletion and type checking at every step of our chain.

Proposal (If applicable)

I have re-written the type declaration of the assign 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> {
  /**
   * Assigns new fields to the dict output of this runnable. Returns a new runnable.
   */
  assign<TAssigned extends Record<string, unknown>>(
    mapping: RunnableMapLike<RunOutput, TAssigned>
  ): Runnable<
    RunInput,
    Omit<RunOutput, keyof TAssigned> & TAssigned,
    CallOptions
  >;
}

[arnaugomez]

I have created a TypeScript playground with the assign method with TypeScript type inference. Playground Link