Improve support of conversational models

[xenova]

This PR adds support for passing chat messages (with "role" and "content" properties) to the text-generation pipeline.

Example: Chat with Xenova/Qwen1.5-0.5B-Chat.

import { pipeline } from '@xenova/transformers';

// Create text-generation pipeline
const generator = await pipeline('text-generation', 'Xenova/Qwen1.5-0.5B-Chat');

// Define the list of messages
const messages = [
    { role: 'system', content: 'You are a helpful assistant.' },
    { role: 'user', content: 'Tell me a funny joke.' }
]

// Generate text
const output = await generator(messages, {
    max_new_tokens: 128,
    do_sample: false,
    return_full_text: false,
})
console.log(output[0].generated_text);
// [
//   { role: 'system', content: 'You are a helpful assistant.' },
//   { role: 'user', content: 'Tell me a funny joke.' },
//   { role: 'assistant', content: "Sure, here's one:\n\nWhy was the math book sad?\n\nBecause it had too many problems.\n\nI hope you found that joke amusing! Do you have any other questions or topics you'd like to discuss?" },
// ]

[HuggingFaceDocBuilderDev]

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