Fix import in forward_backwards.py

Author: ValerianRey

tests/utils/forward_backwards.py

@@ -6,7 +6,7 @@
 from torch.utils._pytree import PyTree, tree_flatten, tree_map
 
 from torchjd.aggregation import Aggregator, Weighting
-from torchjd.autogram._engine import Engine
+from torchjd.autogram import Engine
 from torchjd.autojac import backward