fix: support typetracer in to_packed for IndexedOptionArray (#2912)

* fix: support typetracer in `to_packed` for `IndexedOptionArray`

* fix: don't avoid touching if typetracer

src/awkward/contents/indexedoptionarray.py

@@ -1718,27 +1718,26 @@ def continuation():
             raise AssertionError(result)
 
     def to_packed(self) -> Self:
-        original_index = self._index.raw(self._backend.nplike)
-        is_none = original_index < 0
-        num_none = self._backend.index_nplike.count_nonzero(is_none)
-        if self.parameter("__array__") == "categorical" or self._content.length <= (
-            len(original_index) - num_none
+        index_nplike = self._backend.index_nplike
+        is_none = self._index.data < 0
+        num_none = index_nplike.index_as_shape_item(index_nplike.count_nonzero(is_none))
+        if self.parameter("__array__") == "categorical" or (
+            self._backend.index_nplike.known_data
+            and self._content.length <= (self._index.length - num_none)
         ):
             return ak.contents.IndexedOptionArray(
                 self._index, self._content.to_packed(), parameters=self._parameters
             )
 
         else:
-            new_index = self._backend.index_nplike.empty(
-                len(original_index), dtype=original_index.dtype
-            )
+            new_index = index_nplike.empty(self._index.length, dtype=self._index.dtype)
             new_index[is_none] = -1
-            new_index[~is_none] = self._backend.index_nplike.arange(
-                len(original_index) - num_none,
-                dtype=original_index.dtype,
+            new_index[~is_none] = index_nplike.arange(
+                index_nplike.shape_item_as_index(new_index.size - num_none),
+                dtype=self._index.dtype,
             )
             return ak.contents.IndexedOptionArray(
-                ak.index.Index(new_index, nplike=self._backend.index_nplike),
+                ak.index.Index(new_index, nplike=index_nplike),
                 self.project().to_packed(),
                 parameters=self._parameters,
             )