rohitg00/ai-engineering-from-scratch · error · ValueError

d_model must be divisible by num_heads

Error message

d_model must be divisible by num_heads

What it means

Error "d_model must be divisible by num_heads" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/19-capstone-projects/36-training-loop-eval/code/main.py:82

class LayerNorm(nn.Module):
    def __init__(self, d_model: int, eps: float = 1e-5) -> None:
        super().__init__()
        self.eps = eps
        self.scale = nn.Parameter(torch.ones(d_model))
        self.shift = nn.Parameter(torch.zeros(d_model))

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        mean = x.mean(dim=-1, keepdim=True)
        var = x.var(dim=-1, keepdim=True, unbiased=False)
        return self.scale * (x - mean) / torch.sqrt(var + self.eps) + self.shift


class MultiHeadAttention(nn.Module):
    def __init__(self, cfg: ModelConfig) -> None:
        super().__init__()
        if cfg.d_model % cfg.num_heads != 0:
            raise ValueError("d_model must be divisible by num_heads")
        self.d_model = cfg.d_model
        self.num_heads = cfg.num_heads
        self.head_dim = cfg.d_model // cfg.num_heads
        self.context_length = cfg.context_length
        self.qkv = nn.Linear(cfg.d_model, 3 * cfg.d_model, bias=cfg.use_bias)
        self.out_proj = nn.Linear(cfg.d_model, cfg.d_model, bias=cfg.use_bias)
        self.attn_dropout = nn.Dropout(cfg.dropout)
        self.resid_dropout = nn.Dropout(cfg.dropout)
        mask = torch.triu(
            torch.ones(cfg.context_length, cfg.context_length, dtype=torch.bool),
            diagonal=1,
        )
        self.register_buffer("causal_mask", mask, persistent=False)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        batch, seq, dim = x.shape
        qkv = self.qkv(x)
        q, k, v = qkv.split(self.d_model, dim=-1)

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/19-capstone-projects/36-training-loop-eval/code/main.py:82 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rohitg00/ai-engineering-from-scratch@39ea8a1c6d (2026-08-26). Data as JSON: /api/errors/be7483e8310bdfc7. Report an issue: GitHub.