将输入大小从28更改为784.(784 = 28 * 28).

Change your input size from 28 to 784. (784=28*28).

输入大小自变量是序列中一个元素的特征数量，即一幅mnist图像的特征数量，即像素数即图像的宽度*高.

Input size argument is the number of features in one element of the sequence, so the number of feature of an mnist image, so the number of pixels which is width*hight of the image.