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

@@ -14594,6 +14594,9 @@ defmodule Nx do
 
       iex> Nx.gather(Nx.tensor([[1, 2], [3, 4]]), Nx.tensor([[0, 0]], type: :f32))
       ** (ArgumentError) indices must be an integer tensor, got {:f, 32}
+
+      iex> Nx.gather(Nx.iota({3}), Nx.tensor(0))
+      ** (ArgumentError) expected indices rank to be at least 1, got: 0
   """
   @doc type: :indexed
   def gather(tensor, indices, opts \\ []) do
@@ -14602,6 +14605,10 @@ 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_test.exs

@@ -2863,6 +2863,12 @@ defmodule NxTest do
         Nx.gather(t, Nx.tensor([[0, -1]]))
       end
     end
+
+    test "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
   end
 
   describe "variance/1" do