mlx-examples/llms/mlx_lm/models/mixtral.py

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

import mlx.core as mx
import mlx.nn as nn
import numpy as np

from .base import BaseModelArgs


@dataclass
class ModelArgs(BaseModelArgs):
    model_type: str
    vocab_size: int = 32000
    hidden_size: int = 4096
    intermediate_size: int = 14336
    num_hidden_layers: int = 32
    num_attention_heads: int = 32
    num_experts_per_tok: int = 2
    num_key_value_heads: int = 8
    num_local_experts: int = 8
    rms_norm_eps: float = 1e-5
    rope_theta: float = 1e6
    rope_traditional: bool = False
    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


class MixtralAttention(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.hidden_size = args.hidden_size
        self.num_heads = args.num_attention_heads
        self.head_dim = self.hidden_size // self.num_heads
        self.num_key_value_heads = args.num_key_value_heads
        self.rope_theta = args.rope_theta

        self.scale = self.head_dim**-0.5

        self.q_proj = nn.Linear(
            self.hidden_size, self.num_heads * self.head_dim, bias=False
        )
        self.k_proj = nn.Linear(
            self.hidden_size, self.num_key_value_heads * self.head_dim, bias=False
        )
        self.v_proj = nn.Linear(
            self.hidden_size, self.num_key_value_heads * self.head_dim, bias=False
        )
        self.o_proj = nn.Linear(
            self.num_heads * self.head_dim, self.hidden_size, bias=False
        )

        self.rope = nn.RoPE(
            self.head_dim,
            traditional=args.rope_traditional,
            base=args.rope_theta,
        )

    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.num_heads, -1).transpose(0, 2, 1, 3)
        keys = keys.reshape(B, L, self.num_key_value_heads, -1).transpose(0, 2, 1, 3)
        values = values.reshape(B, L, self.num_key_value_heads, -1).transpose(
            0, 2, 1, 3
        )

        if cache is not None:
            queries = self.rope(queries, offset=cache.offset)
            keys = self.rope(keys, offset=cache.offset)
            keys, values = cache.update_and_fetch(keys, values)
        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)


class MixtralBLockSparseTop2MLP(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.ffn_dim = args.intermediate_size
        self.hidden_dim = args.hidden_size

        self.w1 = nn.Linear(self.hidden_dim, self.ffn_dim, bias=False)
        self.w2 = nn.Linear(self.ffn_dim, self.hidden_dim, bias=False)
        self.w3 = nn.Linear(self.hidden_dim, self.ffn_dim, bias=False)

        self.act_fn = nn.silu

    def __call__(self, x: mx.array) -> mx.array:
        current_hidden_states = self.act_fn(self.w1(x)) * self.w3(x)
        current_hidden_states = self.w2(current_hidden_states)
        return current_hidden_states


class MixtralSparseMoeBlock(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.hidden_dim = args.hidden_size
        self.ffn_dim = args.intermediate_size
        self.num_experts = args.num_local_experts
        self.num_experts_per_tok = args.num_experts_per_tok

        # gating
        self.gate = nn.Linear(self.hidden_dim, self.num_experts, bias=False)

        self.experts = [
            MixtralBLockSparseTop2MLP(args=args) for _ in range(self.num_experts)
        ]

    def __call__(self, x: mx.array) -> mx.array:
        ne = self.num_experts_per_tok
        orig_shape = x.shape
        x = x.reshape(-1, x.shape[-1])

        gates = self.gate(x)

        inds = mx.stop_gradient(mx.argpartition(-gates, kth=ne - 1, axis=-1)[:, :ne])

        scores = mx.softmax(
            mx.take_along_axis(gates, inds, axis=-1).astype(mx.float32),
            axis=-1,
        ).astype(gates.dtype)

        if self.training:
            inds = np.array(inds)
            y = mx.zeros((x.shape[0], ne, x.shape[-1]), x.dtype)
            for e, expert in enumerate(self.experts):
                idx1, idx2 = map(mx.array, np.where(inds == e))
                if idx1.size == 0:
                    continue
                y[idx1, idx2] = expert(x[idx1])

            y = (y * scores[:, :, None]).sum(axis=1)
        else:
            y = []
            for xt, st, it in zip(x, scores, inds.tolist()):
                yt = mx.stack([self.experts[e](xt) for e in it], axis=-1)
                yt = (yt * st).sum(axis=-1)
                y.append(yt[None, :])
            y = mx.concatenate(y)

        return y.reshape(orig_shape)


class MixtralDecoderLayer(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.hidden_size = args.hidden_size

        self.self_attn = MixtralAttention(args)

        self.block_sparse_moe = MixtralSparseMoeBlock(args)
        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
        )

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


class MixtralModel(nn.Module):
    def __init__(self, args: ModelArgs):
        super().__init__()
        self.vocab_size = args.vocab_size
        self.num_hidden_layers = args.num_hidden_layers

        self.embed_tokens = nn.Embedding(args.vocab_size, args.hidden_size)
        self.layers = [
            MixtralDecoderLayer(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
        T = h.shape[1]
        if T > 1:
            mask = nn.MultiHeadAttention.create_additive_causal_mask(T)
            mask = mask.astype(h.dtype)

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

        for layer, c in zip(self.layers, cache):
            h = layer(h, mask, c)

        return self.norm(h)


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

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

    @property
    def layers(self):
        return self.model.layers

    @property
    def head_dim(self):
        return self.args.hidden_size // self.args.num_attention_heads

    @property
    def n_kv_heads(self):
        return self.args.num_key_value_heads