Fix Nx.gather error message for scalar indices

Nx.gather(tensor, scalar) gave an unhelpful Erlang error because
indexed_axes tried to access elem({}, -1) before the shape validation
in Nx.Shape.gather could fire. Moved the scalar check earlier.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: blasphemetheus

nx/lib/nx.ex

@@ -14602,6 +14602,10 @@ defmodule Nx do
     [%T{vectorized_axes: vectorized_axes} = tensor, indices] =
       broadcast_vectors([tensor, indices], align_ranks: false)
 
+    if indices.shape == {} do
+      raise ArgumentError, "expected indices rank to be at least 1, got: 0"
+    end
+
     axes = indexed_axes(tensor, indices, opts)
 
     unless Nx.Type.integer?(indices.type) do

nx/test/nx/gather_scalar_error_test.exs

@@ -0,0 +1,15 @@
+defmodule Nx.GatherScalarErrorTest do
+  use ExUnit.Case, async: true
+
+  test "gather raises correct error on scalar indices" do
+    assert_raise ArgumentError, ~r/expected indices rank to be at least 1/, fn ->
+      Nx.gather(Nx.iota({3}), Nx.tensor(0))
+    end
+  end
+
+  test "gather with valid indices still works" do
+    t = Nx.iota({3, 4})
+    result = Nx.gather(t, Nx.tensor([[0, 0], [2, 3]]))
+    assert Nx.to_flat_list(result) == [0, 11]
+  end
+end