I am still working on my face autoencoder in my spare time, although I have much less spare time lately. My non-variational autoencoder works great - it can very accurately reconstruct any face in my dataset of 400,000 faces, but it doesn't work at all for interpolation or anything like that. So I have also been trying to train a variational autoencoder, but it has a lot more difficulty learning.

For a face which is roughly centered and looking in the general direction of the camera it can do a somewhat decent job, but if the picture is off in any way - there is another face off to the side, there is something blocking the face, the face is at a strange angle, etc it does a pretty bad job. And since I want to try to use this for interpolation training it on these bad faces doesn't really help anything.

One of the biggest datasets I am using is this one from ETHZ. The dataset was created to train a network to predict the age of the person, and while the images are all of good quality it does include many images that have some of the issues I mentioned above, as well as pictures that are not faces at all - like drawings or cartoons. Other datasets I am using consist entirely of properly cropped faces as I described above, but this dataset is almost 200k images, so omitting it completely significantly reduces the size of my training data.

The other day I decided I needed to improve the quality of my training dataset if I ever want to get this variational autoencoder properly trained, and to do that I need to filter out the bad images from the ETHZ IMDB dataset. They had already created the dataset using face detectors, but I want to remove faces that have certain attributes:

• Multiple faces or parts of faces in the image
• Images with something blocking part of the face
• Images where the faces are not generally facing forward, such as profiles

I started trying to curate them manually, but after going through 500 images of the 200k I realized that would not be feasible. It would be easy to train a neural network to classify the faces, but that would require training data, but that still means manually classifying the faces. So, what I did is I took another dataset of faces that were all good and added about 700 bad faces from the IMDB dataset for a total size of about 7000 images and made a new dataset. Then I took a pre-trained discriminator I had previously used as part of a GAN to try to generate faces and retrained it to classify the faces as good or bad. 

I ran this for about 10 epochs, until it was achieving very good accuracy, and then I used it to evaluate the IMDB dataset. Any image which it gave a less than 0.03 probability of being good I moved into the bad training dataset, and any images which it gave a 0.99 probability of being good I moved to the good training dataset. Then I continued training it and so on and so on.

This is called weak supervision or semi-supervised learning, and it works a lot better than I thought it would. After training for a few hours, the images which are moved all seem to be correctly classified, and after each iteration the size of the training dataset grows to allow the network to continue learning. Since I only move images which have very high or very low probabilities, the risk of a misclassification should be relatively low, and I expect to be able to completely sort the IMDB dataset by the end of tomorrow, maybe even sooner. What would have taken weeks or longer to do manually has been reduced to days thanks to transfer learning and weak supervision!

Libellés: coding, data_science, machine_learning, pytorch, autoencoders
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There is one thing that absolutely drives me crazy in Python and that is the fact that you can access a variable that was defined outside of a function from within the function without passing it as an argument. I'm not going to lie, that does come in handy at times - especially when you are working with APIs; but it is still a terrible way to do things.

I like the way scoping is done in C++ - each variable is only valid within the block in which it is declared, but obviously that doesn't work in Python since we don't declare variables. Even so, I think that the only variables that should be available in a function are the ones which are created in it or passed to it as arguments. While we can't modify variables that were declared outside of a function in Python, only access them, if the variable is an object you can modify it using it's methods. So if we have a list and we append to it in a function using list.append() it will actually modify the list, which is crazy.

Being able to access variables that were declared outside of the function makes it easy to write bugs and makes it hard to find them. There have been several times when I have declared a variable outside of a function and then I pass it in to the function with a different name and then use the external variable instead of the one passed into the function. The code will work, but not as expected, and it is difficult to track things like this down.

This makes me see much of the merit in functional programming languages like Scala.

Libellés: coding, python
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I've been working with AWS Lambda recently and I am very impressed. Usually if I need to set up a microservice or a recurring task or anything like that I'll just set up something on one of my virtual servers so I didn't think Lambda would be all that useful. But it makes it really, really easy to set up little tasks and it is much cheaper than having a whole virtual server.

You can create tasks in a number of different languages, and set up a variety of triggers ranging from HTTP requests to scheduled tasks, and when the Lambda is triggered AWS spins it up, executes it and then shuts it down. Since it is so ephemeral it is completely stateless, but you can load files from S3 buckets if you need data of any sort. I assume you can probably also connect to a variety of AWS databases as well, although I haven't done this yet. If you need additional libraries or packages that are not default you can create a layer containing them.

Lambda is not going to replace servers for most use cases, but I think serverless technology is going to make quite a dent in the near future.

Libellés: coding, aws, lambda
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Usually when you think of a gradient boosted decision tree you think of XGBoost or LightGBM. I'd heard of CatBoost but I'd never tried it and it didn't seem too popular. I was looking at a Kaggle competition which had a lot of categorical data and I had squeezed just about every drop of performance I could out of LGBM so I decided to give CatBoost a try. I was extremely impressed.

Out of the box, with all default parameters, CatBoost scored better than the LGBM I had spent about a week tuning. CatBoost trained significantly slower than LGBM, but it will run on a GPU and doing so makes it train just slightly slower than the LGBM. Unlike XGBoost it can handle categorical data, which is nice because in this case we have far too many categories to do one-hot encoding. I've read the documentation several times but I am still unclear as to how exactly it encodes the categorical data, but whatever it does works very well.

I am just beginning to try to tune the hyperparameters so it is unclear how much (if any) extra performance I'll be able to squeeze out of it, but I am very, very impressed with CatBoost and I highly recommend it for any datasets which contain categorical data. Thank you Yandex! 

Libellés: coding, data_science, machine_learning, kaggle, catboost
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The other day I was having problems with a CoLab notebook and I was trying to debug it when I noticed that TPU is now an option for runtime type. I found no references to this in the CoLab documentation, but apparently it was quietly introduced only recently. If anyone doesn't know, TPUs are chips designed by Google specifically for matrix multiplications and are supposedly incredibly fast. Last I checked the cost to rent one through GCP was about $6 per hour, so the ability to have access to one for free could be a huge benefit.

As TPUs are specialized chips you can't just run the same code as on a CPU or a GPU. TPUs do not support all TensorFlow operations and you need to create a special optimizer to be able to take advantage of the TPU at all. The model I was working with at the time was created using TensorFlow's Keras API so I decided to try to convert that to be TPU compatible in order to test it.

Normally you would have to use a cross shard optimizer, but there is a shortcut for Keras models:

TPU_WORKER = 'grpc://' + os.environ['COLAB_TPU_ADDR']

# create network and compiler
tpu_model = tf.contrib.tpu.keras_to_tpu_model(
keras_model, strategy = tf.contrib.tpu.TPUDistributionStrategy(
    tf.contrib.cluster_resolver.TPUClusterResolver(TPU_WORKER)))

The first line finds an available TPU and gets it's address. The second line takes your keras model as input and converts it to a TPU compatible model. Then you would train the model using tpu_model.fit() instead of keras_model. This was the easy part.

For this particular model I am using a lot of custom functions for loss and metrics. Many of the functions turned out to not be compatible with TPUs so had to be rewritten. While at the time this was annoying, it turned out to be worth it regardless of the TPU because I had to optimize the functions in order to make them compatible with TPUs. The specific operations which were not compatible were non-matrix ops - logical operations and boolean masks specifically. Some of the code was downright hideous and this forced me to sit down and think through it and re-write it in a much cleaner manner, vectorizing as much as possible.

After all that effort, so far my experience with the TPUs hasn't been all that great. I can train my model with a significantly larger batch size - whereas  on an Nvidia K80 16 was the maximum batch size, I am currently training with batches of 64 on the TPU and may be able to push that even higher. However the time per epoch hasn't really improved all that much - it is about 1750 seconds on the TPU versus 1850 seconds on the K80. I have read code may need to be altered more to take full advantage of TPUs and I have not really tried playing with the batch size to see how that changes the performance yet.

I suspect that if I did some more research about TPUs and coded the model to be optimized for a TPU from scratch there might be a more noticeable performance gain, but this is based solely on having heard other people talk about how fast they are and not from my experience. 

Update - I have realized that the data augmentation is the bottleneck which is limiting the speed of training. I am training with a Keras generator which performs the augmentation on the CPU and if this is removed or reduced the TPUs do, in fact, train significantly faster than a GPU and also yield better results.

Libellés: coding, machine_learning, google_cloud
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