The world is changing and so is the technology serving it. It’s crucial for everyone to keep up with the rapid changes in technology. One of the domains which is witnessing the fastest and largest evolution is Artificial Intelligence.

We are training our machines to learn and the results are now getting better and better. There are GANs which can generate new images, Deep Learning models for translating signed language into text, and what not! In this swift-moving domain, PyTorch has originated as a new choice for building these models.

Taking into account all the pros of knowing PyTorch, we have decided to write a series of blog posts on Deep Learning with PyTorch. We are going to cover the following topics.

## What is PyTorch?

PyTorch is a Python-based library which facilitates building Deep Learning models and using them in various applications. But, it’s more than just another Deep Learning library, it’s a scientific computing package (as the official PyTorch docs state).

It’s a Python-based

Deep Learning with PyTorch: A 60 Minute Blitzscientific computing packagetargeted at two sets of audiences:

1. A replacement for NumPy to use the power of GPUs

2. A deep learning research platform that provides maximum flexibility and speed

PyTorch uses **Tensor **as its core data structure, which is similar to Numpy array. If you are wondering about this specific choice of data structure, the answer lies in the fact that with appropriate software and hardware available, tensors provide acceleration of various mathematical operations. These operations when carried out in a large number in Deep Learning make a huge difference in speed.

PyTorch, similar to Python, focuses on the ease of use and makes it possible for even users with very basic programming knowledge to use Deep Learning in their projects. This also makes it the perfect “*first deep learning library to learn*“, if you don’t know one already.

## Why should I learn PyTorch?

In the previous section, we mentioned that PyTorch is the perfect choice for the first deep learning library you should learn. In this section, we will elaborate on why so.

There is no shortage of Deep Learning libraries: Keras, Tensorflow, Caffe, Theano (RIP) and many more. But what makes PyTorch different?

An ideal deep learning library should be easy to learn and use, flexible enough to be used in various applications, efficient so that we can deal with huge real-life datasets and accurate enough to provide correct results even in presence of uncertainty in input data.

PyTorch performs really well on all these metrics. The “**pythonic**” coding style makes it simple to learn and use. **GPU acceleration**, support for **distributed computing **and **automatic gradient calculation** helps in performing backward pass automatically starting from a forward expression.

Of course, because of Python, it faces a risk of the slow runtime but the **high-performance C++ API** (libtorch) removes that overhead. This makes the transition from R&D to Production very smooth. One more reason to use PyTorch?

## Overview of the PyTorch Library

Now that we have discussed about PyTorch and what makes it unique, let’s have a look at a basic pipeline of a PyTorch project. The figure below describes a typical workflow along with the important modules associated with each step.

The important PyTorch modules that we are going to briefly discuss here are: **torch.nn**, **torch.optim**, **torch.utils and torch.autograd.**

### 1. Data Loading and Handling

The very first step in any deep learning project deals with data loading and handling. PyTorch provides utilities for the same in **torch.utils.data.**

The two important classes in this module are **Dataset** and **DataLoader**.

**Dataset**is built on top of**Tensor**data type and is used primarily for custom datasets.**DataLoader**is used when you have a large dataset and you want to load data from a**Dataset**in background so that it’s ready and waiting for the training loop.

We can also use **torch.nn.DataParallel** and **torch.distributed** if we have access to multiple machines or GPUs.

### 2. Building Neural Network

The **torch.nn** module is used for creating Neural Networks. It provides all the common neural network layers like fully connected layers, convolutional layers, activation and loss functions etc.

Once the network architecture is created and data is ready to be fed to the network, we need techniques to update the weights and biases so that the network starts to **learn**. These utilities are provided in **torch.optim** module. Similarly, for automatic differentiation which is required during backward pass, we use the **torch.autograd** module.

### 3. Model Inference & Compatibility

After the model has been trained, it can be used to predict output for test cases or even new datasets. This process is referred to as **model inference**.

PyTorch also provides **TorchScript** which can be used to run models independently from a Python runtime. This can be thought of as a Virtual Machine with instructions mainly specific to Tensors.

You can also convert model trained using PyTorch into formats like **ONNX,** which allow you to use these models in other DL frameworks such as MXNet, CNTK, Caffe2. You can also convert **onnx** models to Tensorflow.

## Introduction to Tensors

So far in this post, we have discussed about PyTorch and why should you learn it. Now it’s time to start the very same journey. We will kick this off with Tensors – the core data structure used in PyTorch.

Tensor is simply a fancy name given to matrices. If you are familiar with NumPy arrays, understanding and using PyTorch Tensors will be very easy. A scalar value is represented by a 0-dimensional Tensor. Similarly a column/row matrix using a 1-D Tensor and so on. Some examples of Tensors with different dimensions are shown below to give you a better picture.

Before we start with the introduction to Tensors, let’s install PyTorch 1.1.0

conda install -c pytorch pytorch-cpu

That’s it! Now let’s get started. We suggest you to use **Google Colab** and follow along. Chose a GPU runtime type from the Menu.

### Construct your first Tensor

Let’s see how we can create a PyTorch Tensor. First, we will import PyTorch.

import torch # Create a Tensor with just ones in a column a = torch.ones(5) # Print the tensor we created print(a) # tensor([1., 1., 1., 1., 1.]) # Create a Tensor with just zeros in a column b = torch.zeros(5) print(b) # tensor([0., 0., 0., 0., 0.])

We can similarly create Tensor with custom values as shown below.

c = torch.tensor([1.0, 2.0, 3.0, 4.0, 5.0]) print(c) # tensor([1., 2., 3., 4., 5.])

In all the above cases, we have created vectors or Tensors of dimension 1. Now, let’s create some tensors of higher dimension.

d = torch.zeros(3,2) print(d) # tensor([[0., 0.], # [0., 0.], # [0., 0.]]) e = torch.ones(3,2) print(e) # tensor([[1., 1.], # [1., 1.], # [1., 1.]]) f = torch.tensor([[1.0, 2.0],[3.0, 4.0]]) print(f) # tensor([[1., 2.], # [3., 4.]]) # 3D Tensor g = torch.tensor([[[1., 2.], [3., 4.]], [[5., 6.], [7., 8.]]]) print(g) # tensor([[[1., 2.], # [3., 4.]], # # [[5., 6.], # [7., 8.]]])

We can also find out the shape of a Tensor using .**shape** method.

print(f.shape) # torch.Size([2, 2]) print(e.shape) # torch.Size([3, 2]) print(g.shape) # torch.Size([2, 2, 2])

### Access an element in Tensor

Now that we have created some tensors, let’s see how we can access an element in a Tensor. First let’s see how to do this for 1D Tensor aka vector.

# Get element at index 2 print(c[2]) # tensor(3.)

What about 2D or 3D Tensor? Recall what we mentioned about **dimension of a tensor** in last section. To access one particular element in a tensor, we will need to specify indices equal to the dimension of the tensor. That’s why for tensor **c** we only had to specify one index.

# All indices starting from 0 # Get element at row 1, column 0 print(f[1,0]) # We can also use the following print(f[1][0]) # tensor(3.) # Similarly for 3D Tensor print(g[1,0,0]) print(g[1][0][0]) # tensor(5.)

But what if you wanted to access one entire row in a 2D Tensor? We can use the same syntax as we would use in NumPy Arrays.

# All elements print(f[:]) # All elements from index 1 to 2 (inclusive) print(c[1:3]) # All elements till index 4 (exclusive) print(c[:4]) # First row print(f[0,:]) # Second column print(f[:,1])

### Specify data type of elements

Whenever we create a tensor, PyTorch decides the data type of the elements of the tensor such that the data type can cover all the elements of the tensor. We can override this by specifying the data type while creating the tensor.

int_tensor = torch.tensor([[1,2,3],[4,5,6]]) print(int_tensor.dtype) # torch.int64 # What if we changed any one element to floating point number? int_tensor = torch.tensor([[1,2,3],[4.,5,6]]) print(int_tensor.dtype) # torch.float32 print(int_tensor) # tensor([[1., 2., 3.], # [4., 5., 6.]]) # This can be overridden as follows int_tensor = torch.tensor([[1,2,3],[4.,5,6]], dtype=torch.int32) print(int_tensor.dtype) # torch.int32 print(int_tensor) # tensor([[1, 2, 3], # [4, 5, 6]], dtype=torch.int32)

### Tensor to/from NumPy Array

We have mentioned several times that PyTorch Tensors and NumPy arrays are pretty similar. This of course demands the question if it’s possible to convert one data structure into another. Let’s see how we can do this.

# Import NumPy import numpy as np # Tensor to Array f_numpy = f.numpy() print(f_numpy) # array([[1., 2.], # [3., 4.]], dtype=float32) # Array to Tensor h = np.array([[8,7,6,5],[4,3,2,1]]) h_tensor = torch.from_numpy(h) print(h_tensor) # tensor([[8, 7, 6, 5], # [4, 3, 2, 1]])

### Arithmetic Operations on Tensors

Now it’s time for the next step. Let’s see how we can perform arithmetic operations on PyTorch tensors.

# Create tensor tensor1 = torch.tensor([[1,2,3],[4,5,6]]) tensor2 = torch.tensor([[-1,2,-3],[4,-5,6]]) # Addition print(tensor1+tensor2) # We can also use print(torch.add(tensor1,tensor2)) # tensor([[ 0, 4, 0], # [ 8, 0, 12]]) # Subtraction print(tensor1-tensor2) # We can also use print(torch.sub(tensor1,tensor2)) # tensor([[ 2, 0, 6], # [ 0, 10, 0]]) # Multiplication # Tensor with Scalar print(tensor1 * 2) # tensor([[ 2, 4, 6], # [ 8, 10, 12]]) # Tensor with another tensor # Elementwise Multiplication print(tensor1 * tensor2) # tensor([[ -1, 4, -9], # [ 16, -25, 36]]) # Matrix multiplication tensor3 = torch.tensor([[1,2],[3,4],[5,6]]) print(torch.mm(tensor1,tensor3)) # tensor([[22, 28], # [49, 64]]) # Division # Tensor with scalar print(tensor1/2) # tensor([[0, 1, 1], # [2, 2, 3]]) # Tensor with another tensor # Elementwise division print(tensor1/tensor2) # tensor([[-1, 1, -1], # [ 1, -1, 1]])

### CPU v/s GPU Tensor

PyTorch has different implementation of Tensor for CPU and GPU. Every tensor can be converted to GPU in order to perform massively parallel, fast computations. All operations that will be performed on the tensor will be carried out using GPU-specific routines that come with PyTorch.

If you don’t have access to a GPU, you can perform these examples on Google Colab. Select GPU as Runtime.

Let’s first see how to create a tensor for GPU.

# Create a tensor for CPU # This will occupy CPU RAM tensor_cpu = torch.tensor([[1.0, 2.0], [3.0, 4.0], [5.0, 6.0]], device='cpu') # Create a tensor for GPU # This will occupy GPU RAM tensor_gpu = torch.tensor([[1.0, 2.0], [3.0, 4.0], [5.0, 6.0]], device='cuda')

If you are using Google Colab, focus on the RAM consumption meter in the top right corner and you will see the GPU RAM consumption increase as soon as you create **tensor_gpu**.

Just like tensor creation, the operations performed for CPU and GPU tensors are also different and consume RAM corresponding to the device specified.

# This uses CPU RAM tensor_cpu = tensor_cpu * 5 # This uses GPU RAM # Focus on GPU RAM Consumption tensor_gpu = tensor_gpu * 5

The key point to note here is that no information flows to CPU in the GPU tensor operations (except if we print or access the tensor).

We can move the GPU tensor to CPU and vice versa as shown below.

# Move GPU tensor to CPU tensor_gpu_cpu = tensor_gpu.to(device='cpu') # Move CPU tensor to GPU tensor_cpu_gpu = tensor_cpu.to(device='cuda')

Phew! That’s all folks. Just to recap, in this blog post we discussed PyTorch, its uniqueness and why should you learn it. We also discussed PyTorch workflow and PyTorch Tensor data type in some depth.

In the next blog, we will see how pre-trained models can be used with the help of **TorchVision**.

Want to learn Deep Learning and Computer Vision in depth? OpenCV (in collaboration with LearnOpenCV) is creating 3 computer vision courses and offering 50% discount ( through Kickstarter ) till June 13th, 2019.

## References

- Deep Learning with PyTorch by Eli Stevens, Luca Antiga – Manning Publication
- PyTorch tutorial