fix as_strided_scatter_backward (#87646)
as_strided_scatter's derivative formula was broken - instead of making a "mask" of 1's and 0's, it would effectively make a mask of 1's and uninitialized memory.
Fixes https://github.com/pytorch/pytorch/issues/88105
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87646
Approved by: https://github.com/albanD
