Line 530 in utils.py is too slow with huge datasets

[andreybondarb]

Line 530 in construct_bucket_vb_wc function in utils.py is too slow with huge datasets. It even freezes if dataset is larger than 300k objects.

I propose to change line

forw_corpus = [pad_char_feature] + list(reduce(lambda x, y: x + [pad_char_feature] + y, forw_features)) + [pad_char_feature]

to

forw_corpus = [pad_char_feature]
for forw_feature in forw_features:
   forw_corpus.extend(forw_feature + [pad_char_feature])

Which works considerably faster with no freezes.

[LiyuanLucasLiu]

thanks and fixed in 4f35e0a
PS: a more up-to-date lib is available at https://github.com/LiyuanLucasLiu/Vanilla_NER