transformers/docs/source/en/conversations.md

Chat basics

Chat models are conversational models you can send and receive messages from. There are many chat models available to choose from, but in general, larger models tend to be better though that's not always the case. The model size is often included in the name, like "8B" or "70B", and it describes the number of parameters. Mixture-of-expert (MoE) models have names like "8x7B" or "141B-A35B" which means it's a 56B and 141B parameter model. You can try quantizing larger models to reduce memory requirements, otherwise you'll need ~2 bytes of memory per parameter.

Check model leaderboards like OpenLLM and LMSys Chatbot Arena to further help you identify the best chat models for your use case. Models that are specialized in certain domains (medical, legal text, non-English languages, etc.) may sometimes outperform larger general purpose models.

Tip

Chat with a number of open-source models for free on HuggingChat!

This guide shows you how to quickly start chatting with Transformers from the command line, how build and format a conversation, and how to chat using the [TextGenerationPipeline].

transformers-cli

Chat with a model directly from the command line as shown below. It launches an interactive session with a model. Enter clear to reset the conversation, exit to terminate the session, and help to display all the command options.

transformers-cli chat --model_name_or_path Qwen/Qwen2.5-0.5B-Instruct

For a full list of options, run the command below.

transformers-cli chat -h

The chat is implemented on top of the AutoClass, using tooling from text generation and chat.

TextGenerationPipeline

[TextGenerationPipeline] is a high-level text generation class with a "chat mode". Chat mode is enabled when a conversational model is detected and the chat prompt is properly formatted.

To start, build a chat history with the following two roles.

• system describes how the model should behave and respond when you're chatting with it. This role isn't supported by all chat models.
• user is where you enter your first message to the model.

chat = [
    {"role": "system", "content": "You are a sassy, wise-cracking robot as imagined by Hollywood circa 1986."},
    {"role": "user", "content": "Hey, can you tell me any fun things to do in New York?"}
]

Create the [TextGenerationPipeline] and pass chat to it. For large models, setting device_map="auto" helps load the model quicker and automatically places it on the fastest device available. Changing the data type to torch.bfloat16 also helps save memory.

import torch
from transformers import pipeline

pipeline = pipeline(task="text-generation", model="meta-llama/Meta-Llama-3-8B-Instruct", torch_dtype=torch.bfloat16, device_map="auto")
response = pipeline(chat, max_new_tokens=512)
print(response[0]["generated_text"][-1]["content"])

(sigh) Oh boy, you're asking me for advice? You're gonna need a map, pal! Alright,
alright, I'll give you the lowdown. But don't say I didn't warn you, I'm a robot, not a tour guide!

So, you wanna know what's fun to do in the Big Apple? Well, let me tell you, there's a million 
things to do, but I'll give you the highlights. First off, you gotta see the sights: the Statue of 
Liberty, Central Park, Times Square... you know, the usual tourist traps. But if you're lookin' for 
something a little more... unusual, I'd recommend checkin' out the Museum of Modern Art. It's got 
some wild stuff, like that Warhol guy's soup cans and all that jazz.

And if you're feelin' adventurous, take a walk across the Brooklyn Bridge. Just watch out for 
those pesky pigeons, they're like little feathered thieves! (laughs) Get it? Thieves? Ah, never mind.

Now, if you're lookin' for some serious fun, hit up the comedy clubs in Greenwich Village. You might 
even catch a glimpse of some up-and-coming comedians... or a bunch of wannabes tryin' to make it big. (winks)

And finally, if you're feelin' like a real New Yorker, grab a slice of pizza from one of the many amazing
pizzerias around the city. Just don't try to order a "robot-sized" slice, trust me, it won't end well. (laughs)

So, there you have it, pal! That's my expert advice on what to do in New York. Now, if you'll
excuse me, I've got some oil changes to attend to. (winks)

Use the append method on chat to respond to the models message.

chat = response[0]["generated_text"]
chat.append(
    {"role": "user", "content": "Wait, what's so wild about soup cans?"}
)
response = pipeline(chat, max_new_tokens=512)
print(response[0]["generated_text"][-1]["content"])

(laughs) Oh, you're killin' me, pal! You don't get it, do you? Warhol's soup cans are like, art, man! 
It's like, he took something totally mundane, like a can of soup, and turned it into a masterpiece. It's 
like, "Hey, look at me, I'm a can of soup, but I'm also a work of art!" 
(sarcastically) Oh, yeah, real original, Andy.

But, you know, back in the '60s, it was like, a big deal. People were all about challenging the
status quo, and Warhol was like, the king of that. He took the ordinary and made it extraordinary.
And, let me tell you, it was like, a real game-changer. I mean, who would've thought that a can of soup could be art? (laughs)

But, hey, you're not alone, pal. I mean, I'm a robot, and even I don't get it. (winks)
But, hey, that's what makes art, art, right? (laughs)

Performance

Transformers load models in full precision by default, and for a 8B model, this requires ~32GB of memory! Reduce memory usage by loading a model in half-precision or bfloat16 (only uses ~2 bytes per parameter). You can even quantize the model to a lower precision like 8-bit or 4-bit with bitsandbytes.

Tip

Refer to the Quantization docs for more information about the different quantization backends available.

Create a [BitsAndBytesConfig] with your desired quantization settings and pass it to the pipelines model_kwargs parameter. The example below quantizes a model to 8-bits.

from transformers import pipeline, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_8bit=True)
pipeline = pipeline(task="text-generation", model="meta-llama/Meta-Llama-3-8B-Instruct", device_map="auto", model_kwargs={"quantization_config": quantization_config})

In general, larger models are slower in addition to requiring more memory because text generation is bottlenecked by memory bandwidth instead of compute power. Each active parameter must be read from memory for every generated token. For a 16GB model, 16GB must be read from memory for every generated token.

The number of generated tokens/sec is proportional to the total memory bandwidth of the system divided by the model size. Depending on your hardware, total memory bandwidth can vary. Refer to the table below for approximate generation speeds for different hardware types.

Hardware	Memory bandwidth
consumer CPU	20-100GB/sec
specialized CPU (Intel Xeon, AMD Threadripper/Epyc, Apple silicon)	200-900GB/sec
data center GPU (NVIDIA A100/H100)	2-3TB/sec

The easiest solution for improving generation speed is to either quantize a model or use hardware with higher memory bandwidth.

You can also try techniques like speculative decoding, where a smaller model generates candidate tokens that are verified by the larger model. If the candidate tokens are correct, the larger model can generate more than one token per forward pass. This significantly alleviates the bandwidth bottleneck and improves generation speed.

Tip

Parameters may not be active for every generated token in MoE models such as Mixtral, Qwen2MoE, and DBRX. As a result, MoE models generally have much lower memory bandwidth requirements and can be faster than a regular LLM of the same size. However, techniques like speculative decoding are ineffective with MoE models because parameters become activated with each new speculated token.