20 * 100 sized Variable a_1 expand as 40 * 100 sized Variable a_2, what is the gradient like when back prop from a_2 to a_1?
I am confused about this ,  since I can back prop from a_2 to a_1, so what operation does PyTorch do, to convert from the 40 * 100 sized gradient to 20 * 100 sized gradient?