Added a welcome phrase at the start of Pytorch notebooks (#474)

Author: csainzribera

s1_development_environment/exercise_files/1_Tensors_in_PyTorch.ipynb

@@ -6,6 +6,8 @@
    "source": [
     "# Introduction to Deep Learning with PyTorch\n",
     "\n",
+    "Welcome to the Pytorch Notebooks!\n",
+    "\n",
     "In this notebook, you will get an introduction to [PyTorch](http://pytorch.org/), which is a framework for building and training neural networks (NN). ``PyTorch`` in a lot of ways behaves like the arrays you know and love from Numpy. These Numpy arrays, after all, are just *tensors*. PyTorch takes these tensors and makes it simple to move them to GPUs for the faster processing needed when training neural networks. It also provides a module that automatically calculates gradients (for backpropagation!) and another module specifically for building neural networks. All together, PyTorch ends up being more coherent with **Python** and the ``Numpy/Scipy`` stack compared to *TensorFlow* and other frameworks.\n",
     "\n"
    ]