mlx-examples/llms/mlx_lm/sample_utils.py

# Copyright © 2023-2024 Apple Inc.

from functools import partial

import mlx.core as mx


@partial(mx.compile, inputs=mx.random.state, outputs=mx.random.state)
def top_p_sampling(logits: mx.array, top_p: float, temperature: float) -> mx.array:
    """
    Apply top-p (nucleus) sampling to logits.

    Args:
        logits: The logits from the model's output.
        top_p: The cumulative probability threshold for top-p filtering.
        temperature: Temperature parameter for softmax distribution reshaping.
    Returns:
        token selected based on the top-p criterion.
    """
    # referenced implementation from https://github.com/huggingface/transformers/blob/main/src/transformers/generation/logits_process.py#L449-L460
    probs = mx.softmax(logits * (1 / temperature), axis=-1)

    # sort probs in ascending order
    sorted_indices = mx.argsort(probs, axis=-1)
    sorted_probs = probs[..., sorted_indices.squeeze(0)]

    cumulative_probs = mx.cumsum(sorted_probs, axis=-1)

    # select tokens with cumulative probs below threshold
    top_probs = mx.where(
        cumulative_probs > 1 - top_p,
        sorted_probs,
        0,
    )

    sorted_token = mx.random.categorical(mx.log(top_probs))
    token = sorted_indices.squeeze(0)[sorted_token]

    return token


@partial(mx.compile, inputs=mx.random.state, outputs=mx.random.state)
def categorical_sampling(logits, temp):
    return mx.random.categorical(logits * (1 / temp))