New BartModel (#2745)

* Results same as fairseq
* Wrote a ton of tests
* Struggled with api signatures
* added some docs

src/transformers/modeling_utils.py

@@ -1448,6 +1448,20 @@ class SequenceSummary(nn.Module):
         return output
 
 
+def create_position_ids_from_input_ids(input_ids, padding_idx):
+    """ Replace non-padding symbols with their position numbers. Position numbers begin at
+    padding_idx+1. Padding symbols are ignored. This is modified from fairseq's
+    `utils.make_positions`.
+
+    :param torch.Tensor x:
+    :return torch.Tensor:
+    """
+    # The series of casts and type-conversions here are carefully balanced to both work with ONNX export and XLA.
+    mask = input_ids.ne(padding_idx).int()
+    incremental_indicies = torch.cumsum(mask, dim=1).type_as(mask) * mask
+    return incremental_indicies.long() + padding_idx
+
+
 def prune_linear_layer(layer, index, dim=0):
     """ Prune a linear layer (a model parameters) to keep only entries in index.
         Return the pruned layer as a new layer with requires_grad=True.