Match RNN code and pseudo-code

[ramon-astudillo]

Ensure guide and code match for the RNN exercise. Also add possible improvements for readability and easy comparison between Numpy and Pytorch versions

this affects the file from the guide

https://github.com/LxMLS/lxmls-guide/blob/master/pages/deep_learning/deep_learning_rnn.tex#L75

and could affect followinglxmls-toolkit files

labs/notebooks/non_linear_sequence_classifiers/exercise_1.ipynb
labs/notebooks/non_linear_sequence_classifiers/exercise_2.ipynb
https://github.com/LxMLS/lxmls-toolkit/blob/master/lxmls/deep_learning/numpy_models/rnn.py

and student branch versions thereof.