Simple Considerations for Simple People Building Fancy Neural Networks

Source: Victor Sanh, Hugging Face Blog, February 25, 2021 URL: https://huggingface.co/blog/simple-considerations Author: Victor Sanh (Hugging Face research scientist, author of DistilBERT)

Core practices (overlaps heavily with Karpathy 2019 recipe)

Data first:

"the very first step of building a neural network is to put aside machine learning and simply focus on your data"

Overfit test:

"it is a good habit when you think you have finished implementing to overfit a small batch of examples (16 for instance). If your implementation is (nearly) correct, your model will be able to overfit and remember these examples by displaying a 0-loss (make sure you remove any form of regularization such as weight decay)."

Baselines:

"Start as simple as possible to get a sense of the difficulty of your task and how well standard baselines would perform." "it is sometimes hard to understand if your performance comes from a bug in your model/code or is simply limited by your model's expressiveness"

NLP-specific: tokenization warning

"when you work with language, have a serious look at the outputs of the tokenizers. I can't count the number of lost hours I spent trying to reproduce results (and sometimes my own old results) because something went wrong with the tokenization."

Common implementation errors listed

Wrong indexing ("really the worst")
Forgetting model.eval() or model.zero_grad()
Preprocessing errors
Loss receiving wrong argument type (probabilities vs. logits)
Uniform constant initialization (breaks symmetry)
Parameters not called in forward pass (no gradients)
Learning rate stuck at 0
Suboptimal input truncation

HP tuning advice

"there is no point of launching 1000 runs with different hyperparameters: compare a couple of runs with different hyperparameters to get an idea of which hyperparameters have the highest impact"

"random over a reasonably manually defined grid search is still a tough-to-beat baseline" [re: Bayesian vs random search]

Embeddings freezing (NLP, pre-trained LM fine-tuning)

"in my experience working with pre-trained language models, freezing the embeddings modules to their pre-trained values doesn't affect much the fine-tuning task performance while considerably speeding up the training."

Credence ~65-70% -- specific domain claim, lacks ablation study reference.

External links from this post

Cecelia Shao, "Checklist for Debugging Neural Networks" (2019, KDnuggets/Towards Data Science): 5-section checklist (start simple, confirm loss, check intermediate outputs, diagnose parameters, track work). Thin; largely overlaps with Karpathy recipe and Slavv. Not captured separately -- see those sources instead.
Chase Roberts, "How to unit test machine learning code" (2017, Medium, 4 min): Focuses on software unit testing practices applied to ML models -- testing gradient flow, output shapes, that outputs change when weights change. Spawned mltest library. Not a full debugging guide. Main insight: "The code never crashes, the loss still goes down, it just converges to poor results."
Joel Grus, "Reproducibility in ML as engineering best practices" (Google Slides): Experiment reproducibility / engineering hygiene. Not fetched.
A recipe for Training Neural Networks -- Karpathy (captured in full: karpathy_recipe_training_nn_2019.md)

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