word_language_model: Some suggested arguments for --model are not valid

[cuevasclemente]

In the word_language_model example, the suggestions for a RNN_TANH or RNN_RELU are not valid arguments. It looks like these haven't been valid members of the API since October 2016: pytorch/pytorch@b5d1329, instead the API determines the type of RNN based on the mode argument to the RNN class.

An easy fix would be to check for either RNN_TANH or RNN_RELU and then pass that as the mode for the class, or use the LSTM or GRU models directly as is done currently in the code (by accessing the classes from getattribute(nn, 'LSTM'/'GRU')