Fix train error of ConcatenatedDoc2Vec in doc2vec-IMDB (#1377)

* fix the compatibility between python2 & 3

* require explicit corpus size, epochs for train()

* make all train() calls use explicit count, epochs

* add tests to make sure that ValueError is indeed thrown

* update test

* fix the word2vec's reset_from()

* require explicit corpus size, epochs for train()

* make all train() calls use explicit count, epochs

* fix some error

* fix test error

* fix the train error of ConcatenatedDoc2Vec

* update the ConcatenatedDoc2Vec class

* update the parameters

* rerun all the cells