[s2s distill] allow pegasus-12-12 (#7104)

examples/seq2seq/distillation.py

@@ -472,6 +472,7 @@ LAYERS_TO_COPY = {
         6: [0, 3, 6, 9, 12, 15],
         8: [0, 2, 4, 6, 8, 10, 12, 15],
         9: [0, 1, 3, 5, 7, 9, 11, 13, 15],
+        12: [0, 1, 2, 3, 4, 5, 6, 7, 9, 11, 13, 15],
         16: list(range(16)),
     },
     6: {1: [0], 2: [0, 5], 3: [0, 2, 5], 4: [0, 1, 3, 5], 6: list(range(6))},