Training the Model

Last updated on 2026-07-24 | Edit this page

Overview

Questions

  • What do the loss function and optimizer actually do during training?
  • What happens, step by step, inside the training loop for one batch of data?
  • What is an epoch, and why do we repeat the training loop for several of them?
  • Why is training accuracy not enough to trust on its own?

Objectives

  • Explain the roles of CrossEntropyLoss and AdamW in training MiniParT.
  • Walk through the six steps that happen for every batch during training.
  • Define “epoch” and explain why the model trains over several of them.
  • Explain why training accuracy alone cannot tell you whether the model generalizes.

Run this first

PYTHON 

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
model = model.to(device)

criterion = nn.CrossEntropyLoss()
optimizer = torch.optim.AdamW(model.parameters(), lr=1e-3, weight_decay=1e-4)

epochs = 10

for epoch in range(epochs):
    model.train()
    total_loss = 0
    correct = 0
    total = 0
    
    for batch_x, batch_y in train_loader:
        batch_x, batch_y = batch_x.to(device), batch_y.to(device)
        
        optimizer.zero_grad()
        outputs = model(batch_x)
        loss = criterion(outputs, batch_y)
        
        loss.backward()
        optimizer.step()
        
        total_loss += loss.item()
        _, predicted = outputs.max(1)
        total += batch_y.size(0)
        correct += predicted.eq(batch_y).sum().item()
        
    train_acc = 100. * correct / total
    print(f"Epoch {epoch+1}/{epochs} | Loss: {total_loss/len(train_loader):.4f} | Train Acc: {train_acc:.2f}%")

OUTPUT 

Epoch 1/10 | Loss: 0.6823 | Train Acc: 65.98%
Epoch 2/10 | Loss: 0.6512 | Train Acc: 67.42%
Epoch 3/10 | Loss: 0.6301 | Train Acc: 68.55%
Epoch 4/10 | Loss: 0.6147 | Train Acc: 69.31%
Epoch 5/10 | Loss: 0.6029 | Train Acc: 69.98%
Epoch 6/10 | Loss: 0.5934 | Train Acc: 70.52%
Epoch 7/10 | Loss: 0.5856 | Train Acc: 70.94%
Epoch 8/10 | Loss: 0.5790 | Train Acc: 71.28%
Epoch 9/10 | Loss: 0.5734 | Train Acc: 71.58%
Epoch 10/10 | Loss: 0.5687 | Train Acc: 71.84%

These exact numbers are illustrative - your own run will vary slightly - but they land in the same range as the roughly 0.68 → 0.56 loss and roughly 66% → 72% training accuracy trend used throughout this lesson, including in Evaluating the Model.

Run the block above first, then read on to see what each part does.

The model from Building MiniParT starts out knowing nothing - its weights are randomly initialized. Training is the repeated process of showing it examples, checking how wrong its guesses are, and nudging its weights to be a little less wrong next time - which is exactly what the code above just did. The rest of this episode walks back through it piece by piece.

Setup

PYTHON 

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
model = model.to(device)

criterion = nn.CrossEntropyLoss()
optimizer = torch.optim.AdamW(model.parameters(), lr=1e-3, weight_decay=1e-4)
  • device - trains on a GPU (cuda) if available, since GPUs do this math much faster than a CPU; otherwise falls back to CPU. Either works for a model this small, just at different speeds.
  • criterion (the loss function) - CrossEntropyLoss measures “how wrong was the guess,” comparing the model’s 3 class scores against the true label. Low if confidently correct, high if confidently wrong - think of it as an automatic grader.
  • optimizer - AdamW adjusts the model’s weights based on the loss, like an editor deciding how to nudge every tunable number after seeing the grade.
    • lr=1e-3 (learning rate) - how big a nudge to make each step. Too big and the model overshoots; too small and it learns painfully slowly. 0.001 is a common starting point.
    • weight_decay=1e-4 - mild extra pressure discouraging weights from growing unnecessarily large, another safeguard against over-memorizing.

The training loop

This is the core loop that updates the model’s weights, once per batch, every epoch:

PYTHON 

epochs = 10
for epoch in range(epochs):
    model.train()
    for batch_x, batch_y in train_loader:
        batch_x, batch_y = batch_x.to(device), batch_y.to(device)

        optimizer.zero_grad()
        outputs = model(batch_x)
        loss = criterion(outputs, batch_y)

        loss.backward()
        optimizer.step()

One epoch is one full pass through every batch in the training set; we do 10 (epochs = 10). A single pass isn’t enough - the weights only move a small amount with each nudge, so the model needs to see the training data multiple times, nudging its weights a little further each time, before it settles into a genuinely useful set of weights. As it trains, watch the printed loss: it should generally trend downward epoch over epoch - that’s the sign the model is actually learning something, rather than stuck or diverging.

For every batch of 256 jet pairs:

  1. model.train() - tells the model we’re training, switching on dropout (see Building MiniParT).
  2. optimizer.zero_grad() - clears leftover gradient information from the previous batch, so nudges don’t incorrectly pile on top of each other.
  3. outputs = model(batch_x) - the forward pass: run this batch through the model and get 3 class scores for each example.
  4. loss = criterion(outputs, batch_y) - compares those scores to the true labels and produces one number: how wrong, on average.
  5. loss.backward() - backpropagation: PyTorch works backward through every layer and calculates how much each weight contributed to the error, so we know which direction to nudge each one.
  6. optimizer.step() - applies those nudges, updating every weight a small amount in the direction that should reduce the loss.

Repeat for every batch, for 10 epochs, and the model gradually gets better at telling Hbb, Hcc, and QCD jet pairs apart.

Callout

CrossEntropyLoss treats every example equally by default. As the previous episode noted, this dataset ends up with slightly more QCD examples than Hbb or Hcc after selection, so the model sees somewhat more QCD during training. The imbalance here is small enough not to need special handling, but it’s worth checking in any classification problem with unequal class sizes - left unchecked in a more imbalanced dataset, a model can learn to just favor the larger class.

Watching it learn

PYTHON 

train_acc = 100. * correct / total
print(f"Epoch {epoch+1}/{epochs} | Loss: {total_loss/len(train_loader):.4f} | Train Acc: {train_acc:.2f}%")

After each epoch, we print the average loss and training accuracy (the percentage of training examples the model got right during that epoch). You should generally see loss go down and accuracy go up epoch over epoch - the model visibly learning.

This is training accuracy, measured on data the model has already seen - useful to sanity-check that learning is happening, but not a fair report card. For that, we need data the model has never seen, which the next episode covers.

Challenge

Question

Q: Suppose optimizer.zero_grad() was accidentally left out of the training loop. Would the code raise an error? What would you expect to happen to training over the 10 epochs instead?

A: No error would be raised. Without optimizer.zero_grad(), the gradient information from loss.backward() would accumulate on top of the previous batch’s instead of starting fresh each time. The weight updates in optimizer.step() would then be based on this incorrectly piled-up information, so training would likely become unstable: loss might behave erratically instead of steadily decreasing, and accuracy might fail to improve smoothly across epochs.

Quick recap

  • CrossEntropyLoss grades how wrong the model’s guesses are; AdamW decides how to adjust the model’s weights in response.
  • Each batch: clear old gradients → forward pass → compute loss → backpropagate → update weights.
  • One full pass through all batches is an epoch; we repeat for several epochs so the model keeps improving.
  • Training accuracy is a useful sanity check, but not a fair test - see Evaluating the Model for that.
Key Points
  • CrossEntropyLoss grades how wrong the model’s guesses are; AdamW decides how to adjust the model’s weights in response.
  • Each batch follows the same six steps: clear old gradients, forward pass, compute loss, backpropagate, update weights.
  • One full pass through all batches is an epoch; we repeat for several epochs so the model keeps improving.
  • Training accuracy is a useful sanity check, but not a fair test, since it is measured on data the model has already seen.