The x vector should have c rows. Try this:
> >>> import torch
> >>> row=4; col=7
> >>> A=torch.full((row,col),2.0)
> >>> r,c=A.size()
> >>> r,c
> (4, 7)
> >>> x=torch.arange(c)+1.0
> >>> x
> tensor([1., 2., 3., 4., 5., 6., 7.])
> >>> A*x
> tensor([[ 2.,  4.,  6.,  8., 10., 12., 14.],
>         [ 2.,  4.,  6.,  8., 10., 12., 14.],
>         [ 2.,  4.,  6.,  8., 10., 12., 14.],
>         [ 2.,  4.,  6.,  8., 10., 12., 14.]])

Or, in a more simple way: In A*x, the number of rows for the x vector, should be equal to the number columns of matrix A.