You could set in_channels=3 for self.conv1 or alternatively convert your images to grayscale using:
data_transforms = transforms.Compose([
    transforms.Grayscale(num_output_channels=1),
    transforms.Resize(256),
    transforms.CenterCrop(256),
    transforms.ToTensor(),
    transforms.Normalize(mean=[0.485, 0.456, 0.406],
                         std=[0.229, 0.224, 0.225] )])

PS: I’ve formatted your code for better readability. You can add code snippets using three backticks ```