Fix the flow for the NLP tutorials (#2818)

The
https://pytorch.org/tutorials/beginner/deep_learning_nlp_tutorial.html
is not showing up the main tutorial which has been broken down
into small tutorials. The README provides links to those tutorials.
This PR makes the connection. All the broken down tutorials has
original author's name mentioned for providing credit.

Signed-off-by: Sahdev Zala <[email protected]>

Author: spzala

beginner_source/deep_learning_nlp_tutorial.rst

@@ -1,6 +1,28 @@
 Deep Learning for NLP with Pytorch
 ----------------------------------
 
+These tutorials will walk you through the key ideas of deep learning
+programming using Pytorch. Many of the concepts (such as the computation
+graph abstraction and autograd) are not unique to Pytorch and are
+relevant to any deep learning toolkit out there.
+
+They are focused specifically on NLP for people who have never written
+code in any deep learning framework (e.g, TensorFlow,Theano, Keras, DyNet).
+The tutorials assumes working knowledge of core NLP problems: part-of-speech
+tagging, language modeling, etc. It also assumes familiarity with neural
+networks at the level of an intro AI class (such as one from the Russel and
+Norvig book). Usually, these courses cover the basic backpropagation algorithm
+on feed-forward neural networks, and make the point that they are chains of
+compositions of linearities and non-linearities. This tutorial aims to get
+you started writing deep learning code, given you have this prerequisite
+knowledge.
+
+Note these tutorials are about *models*, not data. For all of the models,
+a few test examples are created with small dimensionality so you can see how
+the weights change as it trains. If you have some real data you want to
+try, you should be able to rip out any of the models from this notebook
+and use them on it.
+
 1. pytorch_tutorial.py
 	Introduction to PyTorch
 	https://pytorch.org/tutorials/beginner/nlp/pytorch_tutorial.html