PyTorch Training Loop

Overview

A correct PyTorch loop has a precise sequence of operations. Getting the order or the modes wrong produces silent bugs (no gradients, dropout active at eval, leaked compute graphs). This skill encodes the canonical, production-ready loop.

When to use

• Writing a training loop from scratch.
• Debugging a model that won't learn or OOMs.
• Reviewing PyTorch training code.

Canonical loop

import torch
from torch.amp import autocast, GradScaler

device = "cuda" if torch.cuda.is_available() else "cpu"
model.to(device)
optimizer = torch.optim.AdamW(model.parameters(), lr=3e-4, weight_decay=0.01)
scaler = GradScaler(enabled=(device == "cuda"))
best_val = float("inf")

for epoch in range(num_epochs):
    # ---- TRAIN ----
    model.train()
    for x, y in train_loader:
        x, y = x.to(device, non_blocking=True), y.to(device, non_blocking=True)
        optimizer.zero_grad(set_to_none=True)
        with autocast(device_type=device, enabled=(device == "cuda")):
            out = model(x)
            loss = criterion(out, y)
        scaler.scale(loss).backward()
        scaler.unscale_(optimizer)
        torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1.0)
        scaler.step(optimizer)
        scaler.update()

    # ---- VALIDATE ----
    model.eval()
    val_loss = 0.0
    with torch.no_grad():
        for x, y in val_loader:
            x, y = x.to(device), y.to(device)
            val_loss += criterion(model(x), y).item() * x.size(0)
    val_loss /= len(val_loader.dataset)

    # ---- CHECKPOINT BEST ----
    if val_loss < best_val:
        best_val = val_loss
        torch.save({"model": model.state_dict(),
                    "optimizer": optimizer.state_dict(),
                    "epoch": epoch}, "best.pt")

Non-negotiable rules

1. model.train() before training, model.eval() before validation/inference (toggles dropout & batchnorm).
2. optimizer.zero_grad() every step — gradients accumulate otherwise.
3. Wrap validation/inference in torch.no_grad() (or inference_mode()) to save memory.
4. Detach when logging: loss.item(), not loss — keeping tensors leaks the graph and OOMs.
5. Clip gradients for RNNs/transformers to prevent explosions.

Reproducibility header

import torch, numpy as np, random
def seed_everything(seed=42):
    random.seed(seed); np.random.seed(seed)
    torch.manual_seed(seed); torch.cuda.manual_seed_all(seed)
    torch.backends.cudnn.deterministic = True
    torch.backends.cudnn.benchmark = False

Pitfalls

• Forgetting `zero_grad` → gradients pile up, training diverges.
• `eval()` never called → dropout/batchnorm corrupt validation metrics.
• Accumulating `total_loss += loss` (tensor, not .item()) → memory explosion.
• DataLoader with `num_workers=0` on large data → CPU-bound; raise workers + pin_memory=True.
• LR too high → NaN loss; see the ml-debugging skill.

Hand-off

A checkpointed model + seed config that experiment-tracking logs and model-serving loads for inference.