models.py

# Copyright © 2023 Apple Inc.

import inspect
import math
from dataclasses import dataclass
from typing import Dict, Optional, Tuple, Union

import mlx.core as mx
import mlx.nn as nn


@dataclass
class ModelArgs:
    hidden_size: int
    num_hidden_layers: int
    intermediate_size: int
    num_attention_heads: int
    rms_norm_eps: float
    vocab_size: int
    num_key_value_heads: int = None
    rope_theta: float = 10000
    rope_traditional: bool = False
    model_type: str = None
    rope_scaling: Optional[Dict[str, Union[float, str]]] = None

    def __post_init__(self):
        if self.num_key_value_heads is None:
            self.num_key_value_heads = self.num_attention_heads

        if self.rope_scaling:
            required_keys = {"factor", "type"}
            if not all(key in self.rope_scaling for key in required_keys):
                raise ValueError(f"rope_scaling must contain keys {required_keys}")

            if self.rope_scaling["type"] != "linear":
                raise ValueError("rope_scaling 'type' currently only supports 'linear'")

    @classmethod
    def from_dict(cls, params):
        return cls(
            **{
                k: v
                for k, v in params.items()
                if k in inspect.signature(cls).parameters
            }
        )


class LoRALinear(nn.Module):
    @staticmethod
    def from_linear(linear: nn.Linear, rank: int = 8):
        # TODO remove when input_dims and output_dims are attributes
        # on linear and quantized linear
        output_dims, input_dims = linear.weight.shape
        if isinstance(linear, nn.QuantizedLinear):
            input_dims *= 32 // linear.bits
        lora_lin = LoRALinear(input_dims, output_dims, rank)
        lora_lin.linear = linear
        return lora_lin

    def to_linear(self):
        linear = self.linear
        bias = "bias" in linear
        weight = linear.weight
        is_quantized = isinstance(linear, nn.QuantizedLinear)

        # Use the same type as the linear weight if not quantized
        dtype = weight.dtype

        if is_quantized:
            dtype = mx.float16
            weight = mx.dequantize(
                weight,
                linear.scales,
                linear.biases,
                linear.group_size,
                linear.bits,
            )
        output_dims, input_dims = weight.shape
        fused_linear = nn.Linear(input_dims, output_dims, bias=bias)

        lora_b = (self.scale * self.lora_b.T).astype(dtype)
        lora_a = self.lora_a.T.astype(dtype)
        fused_linear.weight = weight + lora_b @ lora_a
        if bias:
            fused_linear.bias = linear.bias

        if is_quantized:
            fused_linear = nn.QuantizedLinear.from_linear(
                fused_linear,
                linear.group_size,
                linear.bits,
            )

        return fused_linear

    def __init__(
        self,
        input_dims: int,
        output_dims: int,
        lora_rank: int = 8,
        bias: bool = False,
        scale: float = 20.0,
    ):
        super().__init__()

        # Regular linear layer weights
        self.linear = nn.Linear(input_dims, output_dims, bias=bias)

        # Scale for low-rank update
        self.scale = scale

        # Low rank lora weights
        scale = 1 / math.sqrt(input_dims)
        self.lora_a = mx.random.uniform(
            low=-scale,
            high=scale,
            shape=(input_dims, lora_rank),
        )
        self.lora_b = mx.zeros(shape=(lora_rank, output_dims))

    def __call__(self, x):
        dtype = self.linear.weight.dtype
        if isinstance(self.linear, nn.QuantizedLinear):
            dtype = self.linear.scales.dtype
        y = self.linear(x.astype(dtype))
        z = (x @ self.lora_a) @ self.lora_b
        return y + self.scale * z


class Attention(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()

        dim = args.hidden_size
        self.n_heads = n_heads = args.num_attention_heads
        self.n_kv_heads = n_kv_heads = args.num_key_value_heads

        self.repeats = n_heads // n_kv_heads

        head_dim = args.hidden_size // n_heads
        self.scale = head_dim**-0.5

        self.q_proj = nn.Linear(dim, n_heads * head_dim, bias=False)
        self.k_proj = nn.Linear(dim, n_kv_heads * head_dim, bias=False)
        self.v_proj = nn.Linear(dim, n_kv_heads * head_dim, bias=False)
        self.o_proj = nn.Linear(n_heads * head_dim, dim, bias=False)
        rope_scale = (
            1 / args.rope_scaling["factor"]
            if args.rope_scaling is not None and args.rope_scaling["type"] == "linear"
            else 1
        )
        self.rope = nn.RoPE(
            head_dim,
            traditional=args.rope_traditional,
            base=args.rope_theta,
            scale=rope_scale,
        )

    def __call__(
        self,
        x: mx.array,
        mask: Optional[mx.array] = None,
        cache: Optional[Tuple[mx.array, mx.array]] = None,
    ) -> mx.array:
        B, L, D = x.shape

        queries, keys, values = self.q_proj(x), self.k_proj(x), self.v_proj(x)

        # Prepare the queries, keys and values for the attention computation
        queries = queries.reshape(B, L, self.n_heads, -1).transpose(0, 2, 1, 3)
        keys = keys.reshape(B, L, self.n_kv_heads, -1).transpose(0, 2, 1, 3)
        values = values.reshape(B, L, self.n_kv_heads, -1).transpose(0, 2, 1, 3)

        if cache is not None:
            key_cache, value_cache = cache
            queries = self.rope(queries, offset=key_cache.shape[2])
            keys = self.rope(keys, offset=key_cache.shape[2])
            keys = mx.concatenate([key_cache, keys], axis=2)
            values = mx.concatenate([value_cache, values], axis=2)
        else:
            queries = self.rope(queries)
            keys = self.rope(keys)

        output = mx.fast.scaled_dot_product_attention(
            queries, keys, values, scale=self.scale, mask=mask
        )
        output = output.transpose(0, 2, 1, 3).reshape(B, L, -1)
        return self.o_proj(output), (keys, values)


class MLP(nn.Module):
    def __init__(self, dim, hidden_dim):
        super().__init__()
        self.gate_proj = nn.Linear(dim, hidden_dim, bias=False)
        self.down_proj = nn.Linear(hidden_dim, dim, bias=False)
        self.up_proj = nn.Linear(dim, hidden_dim, bias=False)

    def __call__(self, x) -> mx.array:
        return self.down_proj(nn.silu(self.gate_proj(x)) * self.up_proj(x))


class TransformerBlock(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.num_attention_heads = args.num_attention_heads
        self.hidden_size = args.hidden_size
        self.self_attn = Attention(args)
        self.mlp = MLP(args.hidden_size, args.intermediate_size)
        self.input_layernorm = nn.RMSNorm(args.hidden_size, eps=args.rms_norm_eps)
        self.post_attention_layernorm = nn.RMSNorm(
            args.hidden_size, eps=args.rms_norm_eps
        )
        self.args = args

    def __call__(
        self,
        x: mx.array,
        mask: Optional[mx.array] = None,
        cache: Optional[Tuple[mx.array, mx.array]] = None,
    ) -> mx.array:
        r, cache = self.self_attn(self.input_layernorm(x), mask, cache)
        h = x + r
        r = self.mlp(self.post_attention_layernorm(h))
        out = h + r
        return out, cache


class LlamaModel(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.args = args
        self.vocab_size = args.vocab_size
        self.num_hidden_layers = args.num_hidden_layers
        assert self.vocab_size > 0
        self.embed_tokens = nn.Embedding(args.vocab_size, args.hidden_size)
        self.layers = [
            TransformerBlock(args=args) for _ in range(args.num_hidden_layers)
        ]
        self.norm = nn.RMSNorm(args.hidden_size, eps=args.rms_norm_eps)

    def __call__(
        self,
        inputs: mx.array,
        cache=None,
    ):
        h = self.embed_tokens(inputs)

        mask = None
        if h.shape[1] > 1:
            mask = nn.MultiHeadAttention.create_additive_causal_mask(h.shape[1])
            mask = mask.astype(h.dtype)

        if cache is None:
            cache = [None] * len(self.layers)

        for e, layer in enumerate(self.layers):
            h, cache[e] = layer(h, mask, cache[e])

        return self.norm(h), cache


class Model(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.model = LlamaModel(args)
        self.lm_head = nn.Linear(args.hidden_size, args.vocab_size, bias=False)

    def __call__(
        self,
        inputs: mx.array,
        cache=None,
    ):
        out, cache = self.model(inputs, cache)
        return self.lm_head(out), cache