Writing a PyTorch module[Marc Lelarge]

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# Lesson 4

##Convolutional neural networks
## ... writing a PyTorch module

<br/><br/>
.bold[Marc Lelarge]

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# Deep Learning pipeline

## Dataset and Dataloader + .red[Model] + .grey[Loss and Optimizer] = .grey[Training]

.center[
          <img src="images/lesson1/ml4.png" style="width: 1000px;" />
]

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# Overview of the course:

1- .grey[Course overview: machine learning pipeline]

2- .grey[PyTorch tensors and automatic differentiation]

3- .grey[Classification with deep learning]

4- Convolutional neural networks

* .red[Building block for a model] - in PyTorch: [Convolutional layrers](https://pytorch.org/docs/stable/nn.html#convolution-layers) and [Pooling layers](https://pytorch.org/docs/stable/nn.html#pooling-layers) in `torch.nn`

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# Writing a PyTorch module

We only built once a Neural Network from scratch in [lesson 2](https://github.com/mlelarge/dataflowr/blob/master/PlutonAI/02_basics_PlutonAI_colab.ipynb)

```
model = torch.nn.Sequential(torch.nn.Linear(2, 1))
```

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A Multilayer perceptron (MLP) with a 2 dimension input, 1 dimension output, ReLU activation and one hidden layers of dimensions 50 and a final sigmoid layer can be written as:
```
import torch.nn as nn

model = torch.nn.Sequential(nn.Linear(2, 50), nn.ReLU(), nn.Linear(50,1), nn.Sigmoid())
```

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But for any model of reasonable complexity, the best is to write a sub-class of `torch.nn.Module`.
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# Writing a PyTorch module

To create a module, one has to inherit from the base class `torch.nn.Module` and implement the constructor `__init__(self, ...)` and the forward pass `forward(self, x)`.

Here is the code for our last model:

```
import torch
import torch.nn as nn
*import torch.nn.functional as F

class MLP(nn.Module):
    
    def __init__(self):
        super(MLP, self).__init__()
        self.fc1 = nn.Linear(in_features=2, out_features=50)
        self.fc2 = nn.Linear(in_features=50, out_features=1)
        
    def forward(self,x):
     x = self.fc1(x)
*    x = F.relu(x)
     x = self.fc2(x)
*    return F.sigmoid(x)
```

[difference between `nn.ReLU()` and `F.relu`](https://discuss.pytorch.org/t/whats-the-difference-between-nn-relu-vs-f-relu/27599/2)

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# Writing a PyTorch module

Inheriting from `torch.nn.Module` provides many mechanisms implemented in the superclass.

First, the `(...)` operator is redefined to call the `forward(...)` method and run additional operations, see [ref](https://discuss.pytorch.org/t/any-different-between-model-input-and-model-forward-input/3690).
The forward pass should be executed through this operator and not by calling forward explicitly.

```
model = MLP()
input = torch.empty(12, 2).normal_()
output = model(input)
```

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All `Parameters` added as class attributes are seen by `Module.parameters()`.

As long as you use autograd-compliant operations, the backward pass is implemented automatically.

This is crucial to allow the update of the `Parameters` with your optimizer.

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# Writing a PyTorch module

Define a loss and an optimizer as before:

```
loss_fn = nn.BCELoss()
optimizer = torch.optim.SGD(model.parameters(), lr=learning_rate)
```

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One step of gradient descent is done as before:
```
output = model(x)
loss = loss_fn(output, label)
optimizer.zero_grad()
loss.backward()
optimizer.step()
```

So now is time to construct more complex architectures!

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The end.