model.py

import argparse
from dataclasses import dataclass
from pathlib import Path
from typing import List, Optional, Tuple

import mlx.core as mx
import mlx.nn as nn
import numpy
import numpy as np
from mlx.utils import tree_unflatten
from transformers import BertTokenizer


@dataclass
class ModelArgs:
    dim: int = 768
    num_attention_heads: int = 12
    num_hidden_layers: int = 12
    vocab_size: int = 30522
    attention_probs_dropout_prob: float = 0.1
    hidden_dropout_prob: float = 0.1
    layer_norm_eps: float = 1e-12
    max_position_embeddings: int = 512


model_configs = {
    "bert-base-uncased": ModelArgs(),
    "bert-base-cased": ModelArgs(),
    "bert-large-uncased": ModelArgs(
        dim=1024, num_attention_heads=16, num_hidden_layers=24
    ),
    "bert-large-cased": ModelArgs(
        dim=1024, num_attention_heads=16, num_hidden_layers=24
    ),
}


class TransformerEncoderLayer(nn.Module):
    """
    A transformer encoder layer with (the original BERT) post-normalization.
    """

    def __init__(
        self,
        dims: int,
        num_heads: int,
        mlp_dims: Optional[int] = None,
        layer_norm_eps: float = 1e-12,
    ):
        super().__init__()
        mlp_dims = mlp_dims or dims * 4
        self.attention = nn.MultiHeadAttention(dims, num_heads, bias=True)
        self.ln1 = nn.LayerNorm(dims, eps=layer_norm_eps)
        self.ln2 = nn.LayerNorm(dims, eps=layer_norm_eps)
        self.linear1 = nn.Linear(dims, mlp_dims)
        self.linear2 = nn.Linear(mlp_dims, dims)
        self.gelu = nn.GELU()

    def __call__(self, x, mask):
        attention_out = self.attention(x, x, x, mask)
        add_and_norm = self.ln1(x + attention_out)

        ff = self.linear1(add_and_norm)
        ff_gelu = self.gelu(ff)
        ff_out = self.linear2(ff_gelu)
        x = self.ln2(ff_out + add_and_norm)

        return x


class TransformerEncoder(nn.Module):
    def __init__(
        self, num_layers: int, dims: int, num_heads: int, mlp_dims: Optional[int] = None
    ):
        super().__init__()
        self.layers = [
            TransformerEncoderLayer(dims, num_heads, mlp_dims)
            for i in range(num_layers)
        ]

    def __call__(self, x, mask):
        for layer in self.layers:
            x = layer(x, mask)

        return x


class BertEmbeddings(nn.Module):
    def __init__(self, config: ModelArgs):
        self.word_embeddings = nn.Embedding(config.vocab_size, config.dim)
        self.token_type_embeddings = nn.Embedding(2, config.dim)
        self.position_embeddings = nn.Embedding(
            config.max_position_embeddings, config.dim
        )
        self.norm = nn.LayerNorm(config.dim, eps=config.layer_norm_eps)

    def __call__(self, input_ids: mx.array, token_type_ids: mx.array) -> mx.array:
        words = self.word_embeddings(input_ids)
        position = self.position_embeddings(
            mx.broadcast_to(mx.arange(input_ids.shape[1]), input_ids.shape)
        )
        token_types = self.token_type_embeddings(token_type_ids)

        embeddings = position + words + token_types
        return self.norm(embeddings)


class Bert(nn.Module):
    def __init__(self, config: ModelArgs):
        self.embeddings = BertEmbeddings(config)
        self.encoder = TransformerEncoder(
            num_layers=config.num_hidden_layers,
            dims=config.dim,
            num_heads=config.num_attention_heads,
        )
        self.pooler = nn.Linear(config.dim, config.dim)

    def __call__(
        self,
        input_ids: mx.array,
        token_type_ids: mx.array,
        attention_mask: mx.array = None,
    ) -> Tuple[mx.array, mx.array]:
        x = self.embeddings(input_ids, token_type_ids)

        if attention_mask is not None:
            # convert 0's to -infs, 1's to 0's, and make it broadcastable
            attention_mask = mx.log(attention_mask)
            attention_mask = mx.expand_dims(attention_mask, (1, 2))

        y = self.encoder(x, attention_mask)
        return y, mx.tanh(self.pooler(y[:, 0]))


def load_model(bert_model: str, weights_path: str) -> Tuple[Bert, BertTokenizer]:
    if not Path(weights_path).exists():
        raise ValueError(f"No model weights found in {weights_path}")

    # create and update the model
    model = Bert(model_configs[bert_model])
    model.load_weights(weights_path)

    tokenizer = BertTokenizer.from_pretrained(bert_model)

    return model, tokenizer


def run(bert_model: str, mlx_model: str, batch: List[str]):
    model, tokenizer = load_model(bert_model, mlx_model)

    tokens = tokenizer(batch, return_tensors="np", padding=True)
    tokens = {key: mx.array(v) for key, v in tokens.items()}

    return model(**tokens)


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="Run the BERT model using MLX.")
    parser.add_argument(
        "--bert-model",
        type=str,
        default="bert-base-uncased",
        help="The huggingface name of the BERT model to save.",
    )
    parser.add_argument(
        "--mlx-model",
        type=str,
        default="weights/bert-base-uncased.npz",
        help="The path of the stored MLX BERT weights (npz file).",
    )
    parser.add_argument(
        "--text",
        type=str,
        default="This is an example of BERT working in MLX",
        help="The text to generate embeddings for.",
    )
    args = parser.parse_args()
    run(args.bert_model, args.mlx_model, args.text)