# Output mismatch with channels\_last\_3d?

**URL:** <https://discuss.pytorch.org/t/output-mismatch-with-channels-last-3d/221373>\
**Category:** Memory Format\
**Created:** [July 9, 2025, 3:47am UTC](https://discuss.pytorch.org/t/output-mismatch-with-channels-last-3d/221373 "2025-07-09T03:47:25Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![bartley](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/bartley/32/60569_2.png) [@bartley](https://discuss.pytorch.org/u/bartley)\
**Post date:** [July 9, 2025, 3:47am UTC](https://discuss.pytorch.org/t/output-mismatch-with-channels-last-3d/221373/1 "2025-07-09T03:47:25Z")

</div>

Hi there,

I am trying to implement a convnextv2 model in 3D. I would like the model to support `torch.channels_last_3d` memory format and `torch.compile()`.

I think I am really close but I can not get the input/output to match after conversion. Any ideas?

```python
from types import SimpleNamespace
from functools import partial

import torch
import torch.nn as nn
import torch.nn.functional as F

from timm.layers import trunc_normal_, DropPath

###########
## Utils ##
###########

class LayerNorm3d(nn.Module):
    def __init__ (self, normalized_shape, eps=1e-6, data_format="channels_first"):
        super(). __init__ ()
        self.weight = nn.Parameter(torch.ones(normalized_shape))
        self.bias = nn.Parameter(torch.zeros(normalized_shape))
        self.eps = eps
        self.data_format = data_format
        if self.data_format not in ["channels_last", "channels_first"]:
            raise NotImplementedError 
        self.normalized_shape = (normalized_shape, )
    
    def forward(self, x):
        if self.data_format == "channels_last":
            return F.layer_norm(x, self.normalized_shape, self.weight, self.bias, self.eps)
        elif self.data_format == "channels_first":
            u = x.mean(1, keepdim=True)
            s = (x - u).pow(2).mean(1, keepdim=True)
            x = (x - u) / torch.sqrt(s + self.eps)
            x = self.weight[:, None, None, None] * x + self.bias[:, None, None, None]
            return x

class GRN3d(nn.Module):
    """ 3D GRN (Global Response Normalization) layer
    """
    def __init__ (self, dim):
        super(). __init__ ()
        self.gamma = nn.Parameter(torch.zeros(1, 1, 1, 1, dim))
        self.beta = nn.Parameter(torch.zeros(1, 1, 1, 1, dim))

    def forward(self, x):
        Gx = torch.norm(x, p=2, dim=(1,2,3), keepdim=True)
        Nx = Gx / (Gx.mean(dim=-1, keepdim=True) + 1e-6)
        return self.gamma * (x * Nx) + self.beta + x

############
## Blocks ##
############

class Block(nn.Module):
    """ ConvNeXtV2 Block.
    
    Args:
        dim (int): Number of input channels.
        drop_path (float): Stochastic depth rate. Default: 0.0
    """
    def __init__ (self, dim, drop_path=0.):
        super(). __init__ ()
        self.dwconv = nn.Conv3d(dim, dim, kernel_size=7, padding=3, groups=dim) # depthwise conv
        self.norm = LayerNorm3d(dim, data_format="channels_last")
        self.pwconv1 = nn.Linear(dim, 4 * dim) # pointwise/1x1 convs, implemented with linear layers
        self.act = nn.GELU()
        self.grn = GRN3d(4 * dim)
        self.pwconv2 = nn.Linear(4 * dim, dim)
        self.drop_path = DropPath(drop_path) if drop_path > 0. else nn.Identity()

    def forward(self, x):
        input = x
        x = self.dwconv(x)
        x = x.permute(0, 2, 3, 4, 1) # (N, D, C, H, W) -> (N, D, H, W, C)
        x = self.norm(x)
        x = self.pwconv1(x)
        x = self.act(x)
        x = self.grn(x)
        x = self.pwconv2(x)
        x = x.permute(0, 4, 1, 2, 3) # (N, D, H, W, C) -> (N, C, D, H, W)

        x = input + self.drop_path(x)
        return x

class ConvNeXtV2Encoder3d(nn.Module):
    """ 
    ConvNeXt V2
        
    Args:
        in_chans (int): Number of input image channels. Default: 3
        num_classes (int): Number of classes for classification head. Default: 700
        drop_path_rate (float): Stochastic depth rate. Default: 0.
        head_init_scale (float): Init scaling value for classifier weights and biases. Default: 1.
    """
    def __init__ (
        self, 
        cfg: SimpleNamespace,
        in_chans: int = 3,
        drop_path_rate: float = 0.0,
        head_init_scale: float = 1.0,
        num_classes: float = 700,
    ):
        super(). __init__ ()
        self.cfg= cfg

        # Backbone init cfg
        bb= self.cfg.backbone
        backbone_cfg = {
            "atto": ([2, 2, 6, 2], [40, 80, 160, 320]),
            "femto": ([2, 2, 6, 2], [48, 96, 192, 384]),
            "pico": ([2, 2, 6, 2], [64, 128, 256, 512]),
            "nano": ([2, 2, 8, 2], [80, 160, 320, 640]),
            "tiny": ([3, 3, 9, 3], [96, 192, 384, 768]),
            "base": ([3, 3, 27, 3], [128, 256, 512, 1024]),
            "large": ([3, 3, 27, 3], [192, 384, 768, 1536]),
            "huge": ([3, 3, 27, 3], [352, 704, 1408, 2816]),
        }
        if bb in backbone_cfg:
            depths, dims = backbone_cfg[bb]
        else:
            raise ValueError(f"Backbone: {bb} not implemented.")

        # Downsample layers
        self.downsample_layers = nn.ModuleList() # stem and 3 intermediate downsampling conv layers
        stem = nn.Sequential(
            nn.Conv3d(in_chans, dims[0], kernel_size=4, stride=4),
            LayerNorm3d(dims[0])
        )
        self.downsample_layers.append(stem)
        for i in range(3):
            downsample_layer = nn.Sequential(
                LayerNorm3d(dims[i]),
                nn.Conv3d(
                    dims[i], 
                    dims[i+1], 
                    kernel_size = (2,2,2) if i != 2 else (1,2,2), 
                    stride = (2,2,2) if i != 2 else (1,2,2),
                    ),
            )
            self.downsample_layers.append(downsample_layer)

        # Stages
        self.stages = nn.ModuleList() # 4 feature resolution stages, each consisting of multiple residual blocks
        dp_rates=[x.item() for x in torch.linspace(0, drop_path_rate, sum(depths))] 
        cur = 0
        for i in range(4):
            stage = nn.Sequential(
                *[Block(dim=dims[i], drop_path=dp_rates[cur + j]) for j in range(depths[i])]
            )
            self.stages.append(stage)
            cur += depths[i]

        # Head
        self.norm = LayerNorm3d(dims[-1])
        self.pool = nn.Sequential(
            nn.AdaptiveAvgPool3d((1, 1, 1)),
            nn.Flatten(start_dim=1, end_dim=-1),
        )
        self.head = nn.Linear(dims[-1], num_classes)

        self.apply(partial(self._init_weights, init_type="gelu-based"))
        self.head.weight.data.mul_(head_init_scale)
        self.head.bias.data.mul_(head_init_scale)

    def _init_weights(self, m, init_type="relu-based"):
        if init_type == "relu-based":
            if isinstance(m, (nn.Conv3d, nn.Conv2d, nn.Linear)):
                nn.init.kaiming_normal_(m.weight, mode='fan_out', nonlinearity='relu')
                if m.bias is not None:
                    nn.init.constant_(m.bias, 0)

        elif init_type == "gelu-based":
            if isinstance(m, (nn.Conv3d, nn.Conv2d, nn.Linear)):
                trunc_normal_(m.weight, std=0.02)
                if m.bias is not None:
                    nn.init.constant_(m.bias, 0)

    def forward(self, x):
        for i in range(4):
            x = self.downsample_layers[i](x)
            x = self.stages[i](x)

        x = self.pool(x)
        x = self.norm(x)
        x = self.head(x)
        return x

if __name__ == " __main__":

    def count_parameters(model):
        return sum(p.numel() for p in model.parameters() if p.requires_grad)

    cfg= SimpleNamespace()
    cfg.backbone= "atto"
    cfg.encoder_cfg= SimpleNamespace()

    m = ConvNeXtV2Encoder3d(
        cfg= cfg,
        **vars(cfg.encoder_cfg),
    )
    m.cuda().eval()

    # Param count
    n_params= count_parameters(m)
    print(f"Model: {type(m). __name__ }")
    print("n_param: {:_}".format(n_params))

    # Normal
    x = torch.ones(8, 3, 16, 128, 128).cuda()
    with torch.no_grad():
        z = m.forward(x)
        print(z.shape)

    # Channels Last
    x = x.to(memory_format=torch.channels_last_3d)
    m = m.to(memory_format=torch.channels_last_3d)
    with torch.no_grad():
        z2 = m.forward(x)
        print(z2.shape)

    # Diff
    print("DIFF: {}".format((z - z2).max()))

```

---

<div class="post-metadata">

**Author:** ![bartley](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/bartley/32/60569_2.png) [@bartley](https://discuss.pytorch.org/u/bartley)\
**Post date:** [July 9, 2025, 1:57pm UTC](https://discuss.pytorch.org/t/output-mismatch-with-channels-last-3d/221373/4 "2025-07-09T13:57:29Z")

</div>

The issue was the precision of the `LayerNorm3D` implementation. As the small precision propagates through the network, the difference becomes more pronounced.. Here is the new LayerNorm3d implementation that works with .to(memory\_format=torch.channels\_last\_3d).

```auto
class LayerNorm3d(nn.Module):
    def __init__ (self, normalized_shape, eps=1e-6, data_format="channels_first"):
        super(). __init__ ()
        self.weight = nn.Parameter(torch.ones(normalized_shape))
        self.bias = nn.Parameter(torch.zeros(normalized_shape))
        self.eps = eps
        self.data_format = data_format
        if self.data_format not in ["channels_last", "channels_first"]:
            raise NotImplementedError 
        self.normalized_shape = (normalized_shape, )
    
    def forward(self, x):
        if self.data_format == "channels_last":
            return F.layer_norm(x, self.normalized_shape, self.weight, self.bias, self.eps)
        elif self.data_format == "channels_first":
            return F.layer_norm(
                x.permute(0,2,3,4,1),
                self.normalized_shape,
                self.weight,
                self.bias,
                self.eps,
                ).permute(0,4,1,2,3)

```