add max_relative_length

tensor2tensor/models/transformer_dla.py

@@ -504,4 +504,5 @@ def transformer_dla_rpr_base25():
   hparams = transformer_dla_base()
   hparams.encoder_layers = 25
   hparams.attention_type = "relative_dot_product"
+  hparams.max_relative_length = 20
   return hparams
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