[WIP] Fix length normalization when evaluating on dev set

[neubig]

Currently it seems that activating length normalization will cause crashing on the second epoch when evaluating on the dev set. This adds a unit test to demonstrate the breakage with the following error:

 Start training in minibatch mode...   
 Epoch 1.0000: train_loss/word=3.066 (words=140, words/sec=5200.58, time=0-00:00:00)   
 initialized PolynomialNormalization({'apply_during_search': True, 'm': 1})   
 > Checkpoint   
   Epoch 1.0000 dev [auxiliary] BLEU4: 0   
   Epoch 1.0000 dev Loss: 2.994 (words=140, words/sec=4514.20, time=0-00:00:00)   
   Epoch 1.0000: best dev score, writing model to examples/output/standard.mod   
 Epoch 2.0000: train_loss/word=2.994 (words=140, words/sec=1628.74, time=0-00:00:00)   
 Traceback (most recent call last):
   File "xnmt/xnmt_run_experiments.py", line 166, in <module>
        sys.exit(main())   
   File "xnmt/xnmt_run_experiments.py", line 127, in main
        xnmt_trainer.run_epoch()   
   File "/Users/neubig/work/xnmt/xnmt/xnmt_train.py", line 245, in run_epoch
        self.dev_evaluation()   
   File "/Users/neubig/work/xnmt/xnmt/xnmt_train.py", line 264, in dev_evaluation
        xnmt.xnmt_decode.xnmt_decode(self.decode_args, model_elements=(self.corpus_parser, self.model))   
   File "/Users/neubig/work/xnmt/xnmt/xnmt_decode.py", line 70, in xnmt_decode
        generator.initialize_generator(**args.params_as_dict)   
   File "/Users/neubig/work/xnmt/xnmt/translator.py", line 98, in initialize_generator
        len_norm = xnmt.serializer.YamlSerializer().initialize_object(kwargs["len_norm_type"])   
   File "/Users/neubig/work/xnmt/xnmt/serializer.py", line 51, in initialize_object
        self.set_serialize_params_recursive(deserialized_yaml)   
   File "/Users/neubig/work/xnmt/xnmt/serializer.py", line 79, in set_serialize_params_recursive
        raise ValueError("unknown init parameter for %s: %s" % (obj.yaml_tag, name))   
 ValueError   :    unknown init parameter for !PolynomialNormalization: context   

I'm not sure the cause of this, but perhaps it's related to argument parsing? Any ideas @msperber or @psjanani ?

[msperber]

This should fix it!

[neubig]

Thanks!