transforms.Grayscale()
num_output_channels=1 or 3

import torchvision.transforms as transform
from PIL import Image
import matplotlib.pyplot as plt
import numpy as np
import torch
img0=Image.open('lin-xiao-xun-000003.jpg')
img1=transform.Grayscale(3)(img0)
axs = plt.figure().subplots(1, 2)
axs[0].imshow(img0);axs[0].set_title('src');axs[0].axis('off')
axs[1].imshow(img1,cmap=plt.get_cmap('gray'));axs[1].set_title('gray');axs[1].axis('off')
plt.show()

实图演示：