Repository: OpenNMT/OpenNMT-tf (python)

Issue:
TypeError: 'NoneType' object is not iterable
OpenNMT-tf v2.1.0
Python v3.5.2
tensorflow-gpu v2.0.0

```
model_dir: toy_enru_transformer_withalign

data:
  train_features_file: data/train.en
  train_labels_file: data/train.ru
  train_alignments: data/train.align
  eval_features_file: data/valid.en
  eval_labels_file: data/valid.ru
  source_vocabulary: data/mdl-en.vocab
  target_vocabulary: data/mdl-ru.vocab

params:
  guided_alignment_type: "ce"
  guided_alignment_weight: 1.0

train:
  save_checkpoints_steps: 1000

eval:
  external_evaluators: BLEU
  export_on_best: bleu
  early_stopping:
    metric: bleu
    min_improvement: 0.01
    steps: 4

infer:
  batch_size: 32
```

```
Traceback (most recent call last):
  File "/usr/local/bin/onmt-main", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.5/dist-packages/opennmt/bin/main.py", line 189, in main
    checkpoint_path=args.checkpoint_path)
  File "/usr/local/lib/python3.5/dist-packages/opennmt/runner.py", line 205, in train
    export_on_best=eval_config.get("export_on_best"))
  File "/usr/local/lib/python3.5/dist-packages/opennmt/training.py", line 70, in __call__
    self._model.create_variables(optimizer=self._optimizer)
  File "/usr/local/lib/python3.5/dist-packages/opennmt/models/model.py", line 271, in create_variables
    features = self.examples_inputter.make_features(features=features, training=True)
  File "/usr/local/lib/python3.5/dist-packages/opennmt/models/sequence_to_sequence.py", line 426, in make_features
    element, alignment = element
TypeError: 'NoneType' object is not iterable
```

https://github.com/OpenNMT/OpenNMT-tf/blob/bec38f98de000361b0ce017cc86b9f8d0d2815c9/opennmt/models/model.py#L271

https://github.com/OpenNMT/OpenNMT-tf/blob/bec38f98de000361b0ce017cc86b9f8d0d2815c9/opennmt/models/sequence_to_sequence.py#L433-L435

Trying to unpack a `NoneType` tuple. There's no check for None in `sequence_to_sequence.py` and in `model.py` we don't send a value for `element`.
This looks like a bug. Can you actually train a model on v2.1.0?


Pull request description:
Fix error when creating model variables with guided alignment
Fixes #524.

Patch:
diff --git a/opennmt/models/model.py b/opennmt/models/model.py
index ad68ad80..2dc58e21 100644
--- a/opennmt/models/model.py
+++ b/opennmt/models/model.py
@@ -268,7 +268,7 @@ class Model(tf.keras.layers.Layer):
             [dim or 1 for dim in spec.shape.as_list()[1:]],
             tf.constant("" if spec.dtype is tf.string else 1, dtype=spec.dtype)),
         self.examples_inputter.input_signature())
-    features = self.examples_inputter.make_features(features=features, training=True)
+    features = self.examples_inputter.make_features(features=features)
 
     # Add the batch dimension back before calling the model.
     features, labels = tf.nest.map_structure(lambda x: tf.expand_dims(x, 0), features)


Test patch:
diff --git a/opennmt/tests/model_test.py b/opennmt/tests/model_test.py
index 200b2499..bf6e7926 100644
--- a/opennmt/tests/model_test.py
+++ b/opennmt/tests/model_test.py
@@ -205,6 +205,7 @@ class ModelTest(tf.test.TestCase):
     params["guided_alignment_type"] = ga_type
     features_file, labels_file, data_config = self._makeToyEnDeData(with_alignments=True)
     model.initialize(data_config, params=params)
+    model.create_variables()
     dataset = model.examples_inputter.make_training_dataset(features_file, labels_file, 16)
     features, labels = next(iter(dataset))
     self.assertIn("alignment", labels)