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Python / Python Modern Generative AI and Agents Interview Questions

How do you efficiently load large Hugging Face models for inference, including quantization and device placement?

Loading a 7B+ parameter model naively with from_pretrained() materialises the entire model in FP32 (~28 GB for 7B params), which exceeds most GPU memory budgets. Modern Hugging Face loading uses three key techniques: precision reduction (bfloat16 / float16), device mapping, and on-the-fly quantisation with bitsandbytes.

from transformers import AutoTokenizer, AutoModelForCausalLM
import torch

model_id = 'mistralai/Mistral-7B-Instruct-v0.3'

# ── Option 1: Half precision (BF16) — 2x memory saving, minimal accuracy loss
tokenizer = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(
    model_id,
    torch_dtype=torch.bfloat16,  # half precision
    device_map='auto',           # automatically distribute across GPUs/CPU
)

# ── Option 2: 4-bit quantization with bitsandbytes (QLoRA-style)
from transformers import BitsAndBytesConfig

bnb_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type='nf4',          # NormalFloat4 quantisation
    bnb_4bit_compute_dtype=torch.bfloat16,
    bnb_4bit_use_double_quant=True,     # nested quantisation
)
model_4bit = AutoModelForCausalLM.from_pretrained(
    model_id,
    quantization_config=bnb_config,
    device_map='auto',
)
# 7B model now fits in ~4 GB VRAM

# ── Inference with generate()
messages = [{'role': 'user', 'content': 'What is the capital of France?'}]
inputs = tokenizer.apply_chat_template(
    messages, return_tensors='pt', add_generation_prompt=True
).to(model.device)

with torch.no_grad():
    output_ids = model.generate(
        inputs,
        max_new_tokens=200,
        temperature=0.6,
        do_sample=True,
        pad_token_id=tokenizer.eos_token_id,
    )
# Decode only the generated tokens (not the input prompt)
generated = tokenizer.decode(
    output_ids[0][inputs.shape[1]:], skip_special_tokens=True
)
print(generated)

What does device_map='auto' do when loading a large Hugging Face model?
Approximately how much does 4-bit quantization reduce the VRAM required for a 7B parameter model?

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