Fix BinaryBackend.select_and_scatter crash with :same padding (#1675)Fix/select and scatter negative padding

[blasphemetheus]

Summary

select_and_scatter/8 computes absolute_index in the padded coordinate space but uses it against the unpadded output_shape, producing negative offsets that crash List.duplicate/2.

Fix: subtract low-padding from indices to map back to original coordinates, and filter out indices that land in the padding region.

Changes

nx/lib/nx/binary_backend.ex

• Subtract per-dimension low-padding from absolute_index after computing it in padded space
• Filter output_windows to discard indices outside original tensor bounds

nx/test/nx/defn/grad_test.exs

• Add regression test: grad(sum(window_max(x, {1,3,3,1}, padding: :same, strides: [1,2,2,1]))) on a {1,4,4,1} input — verifies output shape, finiteness, and gradient sum equals number of output elements

Test Results

1170 tests, 0 failures, 1 skipped
1351 doctests, 0 failures

Closes #1675