chore: import upstream snapshot with attribution

decoding/GAD/fairseq/data/prepend_dataset.py

@@ -0,0 +1,28 @@
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+import numpy as np
+import torch
+
+from . import BaseWrapperDataset
+
+
+class PrependDataset(BaseWrapperDataset):
+    def __init__(self, dataset, prepend_getter, ensure_first_token_is=None):
+        super().__init__(dataset)
+        self.prepend_getter = prepend_getter
+        self.ensure_first_token = ensure_first_token_is
+
+    def __getitem__(self, idx):
+        item = self.dataset[idx]
+        is_tuple = isinstance(item, tuple)
+        src = item[0] if is_tuple else item
+
+        assert self.ensure_first_token is None or src[0] == self.ensure_first_token
+        prepend_idx = self.prepend_getter(self.dataset, idx)
+        assert isinstance(prepend_idx, int)
+        src[0] = prepend_idx
+        item = tuple((src,) + item[1:]) if is_tuple else src
+        return item