Deep Learning for Coders / Chapter-8 / Week-10 & Week-11

Published: August 11th, 2021 & August 18th, 2021
Collaborative Filtering
  1. 1.
    Source of the blogpost
  2. 2.
    Re-cap of Chapter-7
  3. 3.
  4. 4.
    Collaborative Filtering
  5. 5.
    Latent Factors
  6. 6.
    Weight Decay
  7. 7.
  8. 8.
    Deep Learning for Collaborative Filtering
  9. 9.
  10. 10.
Source of the blogpost:
This blogpost is based on Chapter-8 from the Fastbook [“DeepLearning for Coders with FastAI & PyTorch”]. And this is created after my participation in Week-10 & Week-11 of FastBook reading session organized by Weight & Biases, hosted by AmanArora.
Re-cap of Chapter-7:
In the previous chapter we have seen several techniques to train a state-of-the-art model. Some of the techniques include normalization, progressive resizing, test time augmentation, mixup and label smoothing.
A normalized data is the one which has mean of 0 and a standard deviation of 1. So it can improve the performance. But what if, we want to gain more performance and on relatively small data. We can start small as needed and then increase the size of the images as needed. This will help the training multiple epochs faster and also understand the issues in an earlier stage. This technique is called progressive resizing. We could also improve performance by augmenting the test set as opposed to the train set. And this technique is called Test time Augmentation.
The another great technique to improve performance is called Mixup, where we pick a random image, a random weight, and then taking weighted average with the selected will give a good mix and it will improve performance. Last but not least we can perform a label smoothing to improve the accuracy in case of one-hot encoded targets.
Recommendation systems are one of the predominant systems in market like Netflix, amazon and Walmart. And also this applies to offline systems such as which product goes in which row to capture the users. And it is one of the challenging problems. The solution for that problem is called Collaborative Filtering.
Collaborative Filtering:
The Collaborative Filtering technique refers to looking at what products the current user has used or liked, find other users that have used or liked similar products, and then recommend other products that those users have used or liked. Lets looks at an example using a MovieLens dataset.
path = untar_data(URLs.ML_100k)
ratings = pd.read_csv(path/'', delimiter='\t', header=None, names=['user','movie','rating','timestamp'])
With this we can now handle the data with in the ratings data frame. We can do an experiment to assign few scores to users and then get the average to multiply that with the movie score but we might end up with a match or not match. This is entirely not right because in first place we need latent factors to know how to score the movies for each user.
Latent Factors:
Latent factors allows us to learn how to score the products for each user set. Below are the steps:
Step 1: Randomly initialize some parameters. These parameters will be a set of latent factors for each user and movie. We will have to decide how many to use.
Step 2: Calculate the predictions. We can do this by simply taking the dot product of each movie with each user. The dot product will be very high for the ones that have a great match otherwise the product will be very low.
Step 3: Calculate the loss using any loss function.
Now after this we can optimize our parameters (that is, the latent factors) using stochastic gradient descent, such as to minimize the loss or in other words the user movies recommendations.
But some times due to the overfitting we can observe that the validation loss get worse and we can use a regularization technique like weight decay.
Weight Decay:
Weight decay also called as L2 regularization, consists of adding sum of all the weights squared to your loss function. When we compute the gradients, it will add a contribution to them that will encourage the weights to be as small as possible and this would prevent overfitting. For a simple function like parabola, we have the following graph.
Screen Shot 2021-08-18 at 4 51 51 PM
Limiting our weights from growing too much is going to complicate the training of the model, but it will yield a state where it generalizes better. To use the weight decay in fastai, just pass wd in your call to fit or fit_one_cycle. And this would yield better results.
model = DotProductBias(n_users, n_movies, 50)
learn = Learner(dls, model, loss_func=MSELossFlat())
learn.fit_one_cycle(5, 5e-3, wd=0.1)
We can create and train a collaborative filtering model using fastAI with below code:
learn = collab_learner(dls, n_factors=50, y_range=(0, 5.5))
The bootstrapping problem is a biggest challenge problem in collaborative filtering and refers to having no users to learn from. One solution would be to gather meta information from users like what genres would they like or what films they would choose from a selected few. One of the main problems in these cases would be the bias that would be introduced initially through the feedback loops. One of the approach that works better with this problem would be the Probabilistic Matrix Factorization/ (PMF) or we could apply deep learning to solve the issues related.
Deep Learning for Collaborative Filtering:
As a first step, we need to concatenate the results of embedding and activations together. This gives us a matrix which we can then pass through linear layers and nonlinearities in the usual way.
Step 1: Getting the embeddings
# Get the embeddings
embs = get_emb_sz(dls)
# Class to create a model by picking up the embeddings
class CollabNN(Module):
def __init__(self, user_sz, item_sz, y_range=(0,5.5), n_act=100):
self.user_factors = Embedding(*user_sz)
self.item_factors = Embedding(*item_sz)
self.layers = nn.Sequential(
nn.Linear(user_sz[1]+item_sz[1], n_act),
nn.Linear(n_act, 1))
self.y_range = y_range
def forward(self, x):
embs = self.user_factors(x[:,0]),self.item_factors(x[:,1])
x = self.layers(, dim=1))
return sigmoid_range(x, *self.y_range)
Step 2: Creating a model with the embeddings
Step 3: Create a Learner and train the model
learn = Learner(dls, model, loss_func=MSELossFlat())
learn.fit_one_cycle(5, 5e-3, wd=0.01)
Step 4: FastAI collab_learner function
learn = collab_learner(dls, use_nn=True, y_range=(0, 5.5), layers=[100,50])
learn.fit_one_cycle(5, 5e-3, wd=0.1)
The learn.model is an object of type EmbeddingNN.
EmbeddingNN allow us to do something very important: we can now directly incorporate other user and movie information, date and time information, or any other information that may be relevant to the recommendation. We now have a brief understanding of how gradient descent can learn intrinsic factors or biases about items from a history of ratings which can provide some insights into the data.
All the code & image used are from fastbook.
Jay Alammar blog on Word2Vec was a great learning resource.