This should work:
class MyDataset(Dataset):
    def __init__(self):
        self.data = torch.randn(100, 3, 24, 24)
        
    def __getitem__(self, index):
        x = self.data[index]
        return x

    def __len__(self):
        return len(self.data)
    

dataset = MyDataset()
loader = DataLoader(
    dataset,
    batch_size=10,
    num_workers=1,
    shuffle=False
)


mean = 0.
std = 0.
nb_samples = 0.
for data in loader:
    batch_samples = data.size(0)
    data = data.view(batch_samples, data.size(1), -1)
    mean += data.mean(2).sum(0)
    std += data.std(2).sum(0)
    nb_samples += batch_samples

mean /= nb_samples
std /= nb_samples

If you can load all samples directly into your RAM, the code will be a bit shorter, but I just assume that’s not possible.