Add image token cost calculation utility for OpenAI

[Magnuti]

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

OpenAI provides a method for estimating the number of tokens associated with image inputs. This calculation can be implemented based on OpenAI's official documentation:

• https://platform.openai.com/docs/guides/images-vision?api-mode=chat#calculating-costs
• https://learn.microsoft.com/en-us/azure/ai-services/openai/overview#image-tokens

We've implemented this logic in our production environment and would like to contribute it to LangChain to make it easier for others to reuse.

Would libs/langchain-openai/src/utils/tools.ts be the correct place to add this utility function?

Motivation

Developers working with image inputs currently need to implement cost calculations themselves. Adding this utility would save time and ensure consistency.

Proposal (If applicable)

Add a method like this that returns the number of tokens for a given image size.

function calculateNumberOfImageTokens(width: Int, height, resolution: "auto" | "low" | "high", model: string): number {}

[tnoori98]

Sounds like a useful utility idea. Would it make sense to turn this into an issue? I'd be happy to help implement it, just need to double check the OpenAI docs for the calculation logic.

[Magnuti]

Relevant PR that somewhat handles image cost calculation #8341