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

hidden must divide by heads

Error message

hidden must divide by heads

What it means

Error "hidden must divide by heads" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/19-capstone-projects/39-instruction-tuning-sft/code/main.py:86

        ids = [self.INST_ID] + inst_bytes + [self.RESP_ID]
        return ids

    def decode_response(self, ids: Sequence[int]) -> str:
        """Decode a generated response, dropping specials."""
        chunk = bytes(i for i in ids if i < 256)
        return chunk.decode("utf-8", errors="replace")


# ---------------------------------------------------------------------------
# Tiny GPT
# ---------------------------------------------------------------------------


class CausalSelfAttention(nn.Module):
    def __init__(self, hidden: int, heads: int, max_len: int):
        super().__init__()
        if hidden % heads != 0:
            raise ValueError("hidden must divide by heads")
        self.heads = heads
        self.head_dim = hidden // heads
        self.qkv = nn.Linear(hidden, hidden * 3, bias=False)
        self.out = nn.Linear(hidden, hidden, bias=False)
        mask = torch.tril(torch.ones(max_len, max_len, dtype=torch.bool))
        self.register_buffer("causal_mask", mask, persistent=False)

    def forward(self, x: torch.Tensor, key_pad_mask: Optional[torch.Tensor] = None) -> torch.Tensor:
        B, T, D = x.shape
        qkv = self.qkv(x).view(B, T, 3, self.heads, self.head_dim).permute(2, 0, 3, 1, 4)
        q, k, v = qkv[0], qkv[1], qkv[2]
        att = (q @ k.transpose(-2, -1)) / math.sqrt(self.head_dim)
        causal = self.causal_mask[:T, :T].view(1, 1, T, T)
        att = att.masked_fill(~causal, float("-inf"))
        if key_pad_mask is not None:
            # key_pad_mask: B x T, 1 for real, 0 for pad.
            km = key_pad_mask.view(B, 1, 1, T).to(torch.bool)
            att = att.masked_fill(~km, float("-inf"))

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/19-capstone-projects/39-instruction-tuning-sft/code/main.py:86 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/28ca4056dca20439. Report an issue: GitHub.