Mismatch in the number of attribution slices for 3D volume Torch tensor

[vggls]

I have a custom Conv3D model written in PyTorch and I want to calculate the GradCAM attributions for a 3D volume (representing an mri). The input tensor has input size of (30, 128, 128) and as per below code the attributions have printed shape of (1, 128, 128, 8). I would like your assistance on the mismatch in the number of calculated attribution slices, as I was expecting 30 (instead of 8).
You may run the following code for your reference.

import torch
import torch.nn as nn
from pytorch_grad_cam import GradCAM #might need to pip install grad-cam

class Conv3D(nn.Module):

    def __init__(self):
        super(Conv3D, self).__init__()

        self.num_labels = 2
        self.classifier = nn.Linear(512, self.num_labels)
        self.dropout = nn.Dropout(0.3)
        self.in_channels = 1
        self.group1 = nn.Sequential(
            nn.Conv3d(self.in_channels, 64, kernel_size=3, padding=1),
            nn.BatchNorm3d(64),
            nn.ReLU(),
            nn.MaxPool3d(kernel_size=(2, 2, 2), stride=(1, 2, 2)))
        self.group2 = nn.Sequential(
            nn.Conv3d(64, 128, kernel_size=3, padding=1),
            nn.BatchNorm3d(128),
            nn.ReLU(),
            nn.MaxPool3d(kernel_size=(2, 2, 2), stride=(2, 2, 2)))
        self.group3 = nn.Sequential(
            nn.Conv3d(128, 256, kernel_size=3, padding=1),
            nn.BatchNorm3d(256),
            nn.ReLU(),
            nn.MaxPool3d(kernel_size=(2, 2, 2), stride=(2, 2, 2)))
        self.group4 = nn.Sequential(
            nn.Conv3d(256, 512, kernel_size=3, padding=1),
            nn.BatchNorm3d(512),
            nn.ReLU(),
            nn.MaxPool3d(kernel_size=(2, 2, 2), stride=(2, 2, 2)))
        self.group5 = nn.Sequential(
            nn.Conv3d(512, 512, kernel_size=3, padding=1),
            nn.BatchNorm3d(512),
            nn.ReLU(),
            nn.MaxPool3d(kernel_size=(1, 2, 2), stride=(2, 2, 2), padding=(0, 1, 1)))

    def forward(self, x):
        print('input', x.shape)
        out = self.group1(x.float()); print('group1', out.shape)
        out = self.group2(out); print('group2', out.shape)
        out = self.group3(out); print('group3', out.shape)
        out = self.group4(out); print('group4', out.shape)
        x = self.group5(out); print('group5', x.shape)
        y = torch.mean(x.view(x.size(0), x.size(1), -1), dim=2); print('avg layer', y.shape)
        logits = self.classifier(y); print('logits', logits.shape)

        return logits
    
model = Conv3D()
mri = torch.rand(30, 128, 128) #a hypothetical mri
 
cam_instance = GradCAM(model=model,target_layers=[model.group5[0]])
input_tensor = mri.unsqueeze(0).unsqueeze(0) #shape is (1,1,30,128,128)
pixel_attributions = cam_instance(input_tensor=input_tensor)
print()
print('pixel attributions', pixel_attributions.shape) #(1, 128, 128, 8)

It will print:

input torch.Size([1, 1, 30, 128, 128])
group1 torch.Size([1, 64, 29, 64, 64])
group2 torch.Size([1, 128, 14, 32, 32])
group3 torch.Size([1, 256, 7, 16, 16])
group4 torch.Size([1, 512, 3, 8, 8])
group5 torch.Size([1, 512, 2, 5, 5])
avg layer torch.Size([1, 512])
logits torch.Size([1, 2])

pixel attributions (1, 128, 128, 8)