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Multi-GPU inference

Built-in Tensor Parallelism (TP) is now available with certain models using PyTorch. Tensor parallelism shards a model onto multiple GPUs, enabling larger model sizes, and parallelizes computations such as matrix multiplication.

To enable tensor parallel, pass the argument `tp_plan="auto"` to from_pretrained():

    import os
    import torch
    from transformers import AutoModelForCausalLM, AutoTokenizer

    model_id = "meta-llama/Meta-Llama-3-8B-Instruct"

    # Initialize distributed
    rank = int(os.environ["RANK"])
    device = torch.device(f"cuda:{rank}")
    torch.distributed.init_process_group("nccl", device_id=device)

    # Retrieve tensor parallel model
    model = AutoModelForCausalLM.from_pretrained(
        model_id,
        tp_plan="auto",
    )

    # Prepare input tokens
    tokenizer = AutoTokenizer.from_pretrained(model_id)
    prompt = "Can I help"
    inputs = tokenizer(prompt, return_tensors="pt").input_ids.to(device)

    # Distributed run
    outputs = model(inputs)

You can use `torchrun` to launch the above script with multiple processes, each mapping to a GPU:

    torchrun --nproc-per-node 4 demo.py

PyTorch tensor parallel is currently supported for the following models:

- Llama

You can request to add tensor parallel support for another model by opening a GitHub Issue or Pull Request.

###
Expected speedups

You can benefit from considerable speedups for inference, especially for inputs with large batch size or long sequences.

For a single forward pass on Llama with a sequence length of 512 and various batch sizes, the expected speedup is as follows:

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