Hi, @Ramzy_Karam -san,
Are you still on issue of;
RuntimeError: size mismatch, m1: [64 x 4], m2: [64 x 512] at c:\a\w\1\s\windows\pytorch\aten\src\th\generic/THTensorMath.cpp:940

If you are so, then, you must understand about matrix operation.
The matrix-matrix multiplication used in fully-connected layer can be written as follows;
for i in range(I)
  for j in range(J)
     c[i][j] = 0.
     for k in range(K)
       c[i][j] += a[i][k]*b[k][j]

You can see the relationship of indexes on each matrices. And you can compare shape of m1 and m2 in the error message.