fix: device in window_scatter_function now uses from_nx (#1640)

Author: Chapaman

torchx/lib/torchx/backend.ex

@@ -1528,13 +1528,15 @@ defmodule Torchx.Backend do
       |> then(unfold_flat)
       |> then(function)
 
+    {device, _} = from_nx(tensor)
+
     indices_to_flatten =
       tensor
       |> Nx.axes()
       |> Enum.map(fn axis ->
         tensor
         |> Nx.shape()
-        |> Nx.iota(axis: axis, backend: Torchx.Backend)
+        |> Nx.iota(axis: axis, backend: {Torchx.Backend, device: device})
         |> then(unfold_flat)
         |> Nx.take_along_axis(Nx.new_axis(arg_idx, -1), axis: -1)
       end)

torchx/test/torchx/device_test.exs

@@ -45,4 +45,12 @@ defmodule Torchx.DeviceTest do
       # assert_raise ArgumentError, fn -> Nx.backend_transfer(t) end
     end
   end
+
+  describe "indices_to_flatten" do
+    test "works" do
+      t = Nx.tensor([[1, 2], [3, 4]], backend: {TB, device: @device})
+      t2 = Nx.tensor([[2, 6], [3, 1]], backend: {TB, device: @device})
+      assert_equal Nx.window_scatter_max(t, t2, 0, {2, 3}), Nx.tensor([[0, 0, 0, 0, 6, 0], [0, 0, 2, 0, 0, 0], [0, 0, 3, 0, 0, 0], [0, 0, 0, 0, 0, 1]], backend: {TB, device: @device})
+    end
+  end
 end