fix: correct type definitions for AutoTokenizer and AutoModelForCausalLM

[VedantMadane]

Description

This PR fixes incorrect TypeScript type definitions for AutoTokenizer and AutoModelForCausalLM.

Fixes #1495

Changes

src/models.js

• PretrainedMixin.from_pretrained: Replaced @type {typeof PreTrainedModel.from_pretrained} with full JSDoc documentation including proper parameter and return type annotations
• AutoModelForCausalLM: Added explicit from_pretrained override with proper JSDoc to ensure TypeScript correctly types the return value

src/tokenizers.js

• PreTrainedTokenizer.from_pretrained: Changed @param {PretrainedTokenizerOptions} options to @param {PretrainedTokenizerOptions} [options] to correctly mark as optional
• AutoTokenizer.from_pretrained: Same fix - marked options parameter as optional

Type Generation

After running npm run typegen, the generated .d.ts files now correctly show:

• from_pretrained methods with optional options parameter (?)
• Proper return types (Promise<PreTrainedTokenizer> and Promise<PreTrainedModel>)

Testing

Created a TypeScript test file to validate the fixes - compilation succeeded with no errors.

[VedantMadane]

Friendly ping - is there anything I can do to help move this PR forward? Happy to make any changes if needed.