src/musubi_tuner/ltx_2/model/common/normalization.py

from enum import Enum

import torch
from torch import nn


class NormType(Enum):
    """Normalization layer types: GROUP (GroupNorm) or PIXEL (per-location RMS norm)."""

    GROUP = "group"
    PIXEL = "pixel"


class PixelNorm(nn.Module):
    """
    Per-pixel (per-location) RMS normalization layer.
    For each element along the chosen dimension, this layer normalizes the tensor
    by the root-mean-square of its values across that dimension:
        y = x / sqrt(mean(x^2, dim=dim, keepdim=True) + eps)
    """

    def __init__(self, dim: int = 1, eps: float = 1e-8) -> None:
        """
        Args:
            dim: Dimension along which to compute the RMS (typically channels).
            eps: Small constant added for numerical stability.
        """
        super().__init__()
        self.dim = dim
        self.eps = eps

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        """
        Apply RMS normalization along the configured dimension.
        """
        # Compute mean of squared values along `dim`, keep dimensions for broadcasting.
        mean_sq = torch.mean(x**2, dim=self.dim, keepdim=True)
        # Normalize by the root-mean-square (RMS).
        rms = torch.sqrt(mean_sq + self.eps)
        return x / rms


def build_normalization_layer(
    in_channels: int, *, num_groups: int = 32, normtype: NormType = NormType.GROUP
) -> nn.Module:
    """
    Create a normalization layer based on the normalization type.
    Args:
        in_channels: Number of input channels
        num_groups: Number of groups for group normalization
        normtype: Type of normalization: "group" or "pixel"
    Returns:
        A normalization layer
    """
    if normtype == NormType.GROUP:
        return torch.nn.GroupNorm(num_groups=num_groups, num_channels=in_channels, eps=1e-6, affine=True)
    if normtype == NormType.PIXEL:
        return PixelNorm(dim=1, eps=1e-6)
    raise ValueError(f"Invalid normalization type: {normtype}")