Compute the mean and std value of dataset.

def get_mean_and_std(dataset):
    """Compute the mean and std value of dataset."""
    dataloader = torch.utils.data.DataLoader(
        dataset, batch_size=1, shuffle=True, num_workers=2
    )
    mean = torch.zeros(3)
    std = torch.zeros(3)
    print("==> Computing mean and std..")
    for inputs, _ in dataloader:
        for i in range(3):
            mean[i] += inputs[:, i, :, :].mean()
            std[i] += inputs[:, i, :, :].std()
    mean.div_(len(dataset))
    std.div_(len(dataset))
    return mean, std