As the title clearly describes, the images in the dataset I use do have 3 color channels despite they are grayscale. So, I used transforms.Grayscale(num_output_channels=1) transformation in order to reduce the number of color channels to 1, but still the loaded images do have 3 channels.
Here is my implementation:
data_transforms = transforms.Compose([
    transforms.Grayscale(num_output_channels=1),
    transforms.ToTensor(),
    transforms.Normalize(mean=[0.485, 0.456, 0.406],
                         std=[0.229, 0.224, 0.225])
])
face_train_dataset = datasets.ImageFolder(root=DEST_PATH_TRAIN, transform=data_transforms)

train_loader = DataLoader(face_train_dataset,
                          batch_size=train_batch_size, shuffle=True, num_workers=4)