Avoid out-of-bound position ids with left padded generation input

lib/bumblebee/text/generation.ex

@@ -290,7 +290,11 @@ defmodule Bumblebee.Text.Generation do
     position_ids =
       attention_mask
       |> Nx.cumulative_sum(axis: 1)
-      |> Nx.subtract(1)
+      # Position ids are zero-indexed, so we want to subtract 1.
+      # However, attention mask may have zeros on the left, in which
+      # case cumulative sum leaves zeros there as well and we don't
+      # want to subtract from these.
+      |> Nx.subtract(Nx.select(attention_mask, 1, 0))
 
     inputs =
       inputs