PyTorch Training

Model-building conventions

• Set bias=False on any nn.Linear/nn.Conv2d immediately followed by BatchNorm — BN's mean-subtraction + learnable beta cancels the bias, making it dead params. Keep bias=True (default) on the output/classifier head, which has no BN after it.

Neural net training checklist (common mistakes if skipped)

Run through these before and during any training run — each one catches a class of silent bug that wastes full training runs:

1. Overfit a single batch first. Before full training, verify the model can drive loss to ~0 on one small batch — if it can't, there's a bug in the model/loss/data pipeline; no point training on the full set.
2. Verify loss @ init. Check the starting loss equals the theoretical value — softmax over n classes should give -log(1/n) (e.g. ~2.30 for 10 classes). A mismatch means a bad head init or mislabeled targets.
3. Input-independent baseline. Train once with inputs zeroed out; the model must do worse than with real inputs. If they match, the data pipeline isn't actually feeding the model (it's only learning the prior).
4. Init the output-layer bias to data statistics. Regression with mean 50 → init bias 50; class imbalance 1:10 → set logit bias so p≈0.1 at init. Kills the "hockey stick" loss curve where early steps just learn the bias.
5. Visualize the exact tensor right before y_hat = model(x). Decode what actually enters the net — the only source of truth for catching preprocessing/augmentation bugs (e.g. forgetting to flip labels when flipping images).
6. Don't trust LR-decay defaults; use a constant LR and tune it last. Borrowed code often decays by epoch number (e.g. ImageNet decays at epoch 30) — on a smaller dataset this silently drives the LR to ~0 before the model converges.