docs目录中的有关save_image的代码组合的错误

[ZoruaSama]

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在torchvision.utils.save_image.md文件中的make_grid函数中：
原来使用size返回的是张量的单元数量，是一个shape为[1]的int64的张量，不是图像的通道数。因此这个代码无法正确运行。
应该将里面的size(0)改为shape[0]，size(1)改为shape[1]。

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