Helps organize PyTorch model training with LightningModule and Trainer, including validation, logging, model saving, and early stopping. It also guides distributed training across multiple GPUs and settings for reduced-precision computation.
Clean training loops with built-in distributed support.
This skill organizes PyTorch model training around LightningModule and Trainer. It brings training logic and optimization settings together, adding validation, testing, logging, and model checkpoints during training. It also covers distributed training across multiple GPUs, mixed precision to adjust computation precision, and gradient accumulation.
Use it when you want to organize a training loop written by hand or expand training from a single GPU to multiple GPUs. It helps you save models based on validation results, stop training early when improvement stalls, and configure learning rate changes. The material also includes examples of checks for situations where memory runs out or validation does not run.
LightningModule and Trainer.”DDP across eight GPUs.”The listed dependencies are lightning, torch, and transformers. The skill covers CPU, GPU, TPU, and Apple MPS, but the distributed training strategy and computation precision must be chosen to match the hardware you use.
This overview was written by AI (OpenAI Codex) from the public SKILL.md. Check the original below for the exact behavior.
AI rated this skill on four points using only the contents of its SKILL.md. Stars and popularity are not taken into account.
The total counts each point as A = 25, B = 15, C = 5. Rated by AI (OpenAI Codex) as of Oct 20, 2026. Treat it as a guide and read the original before installing.
Review the instructions the agent will receive before you install.
PyTorch Lightning organizes PyTorch code to eliminate boilerplate while maintaining flexibility.
Installation:
pip install lightning
Convert PyTorch to Lightning (3 steps):
import lightning as L
import torch
from torch import nn
from torch.utils.data import DataLoader, Dataset
# Step 1: Define LightningModule (organize your PyTorch code)
class LitModel(L.LightningModule):
def __init__(self, hidden_size=128):
super().__init__()
self.model = nn.Sequential(
nn.Linear(28 * 28, hidden_size),
nn.ReLU(),
nn.Linear(hidden_size, 10)
)
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = nn.functional.cross_entropy(y_hat, y)
self.log('train_loss', loss) # Auto-logged to TensorBoard
return loss
def configure_optimizers(self):
return torch.optim.Adam(self.parameters(), lr=1e-3)
# Step 2: Create data
train_loader = DataLoader(train_dataset, batch_size=32)
# Step 3: Train with Trainer (handles everything else!)
trainer = L.Trainer(max_epochs=10, accelerator='gpu', devices=2)
model = LitModel()
trainer.fit(model, train_loader)
That's it! Trainer handles:
Original PyTorch code:
model = MyModel()
optimizer = torch.optim.Adam(model.parameters())
model.to('cuda')
for epoch in range(max_epochs):
for batch in train_loader:
batch = batch.to('cuda')
optimizer.zero_grad()
loss = model(batch)
loss.backward()
optimizer.step()
Lightning version:
class LitModel(L.LightningModule):
def __init__(self):
super().__init__()
self.model = MyModel()
def training_step(self, batch, batch_idx):
loss = self.model(batch) # No .to('cuda') needed!
return loss
def configure_optimizers(self):
return torch.optim.Adam(self.parameters())
# Train
trainer = L.Trainer(max_epochs=10, accelerator='gpu')
trainer.fit(LitModel(), train_loader)
Benefits: 40+ lines → 15 lines, no device management, automatic distributed
class LitModel(L.LightningModule):
def __init__(self):
super().__init__()
self.model = MyModel()
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = nn.functional.cross_entropy(y_hat, y)
self.log('train_loss', loss)
return loss
def validation_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
val_loss = nn.functional.cross_entropy(y_hat, y)
acc = (y_hat.argmax(dim=1) == y).float().mean()
self.log('val_loss', val_loss)
self.log('val_acc', acc)
def test_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
test_loss = nn.functional.cross_entropy(y_hat, y)
self.log('test_loss', test_loss)
def configure_optimizers(self):
return torch.optim.Adam(self.parameters(), lr=1e-3)
# Train with validation
trainer = L.Trainer(max_epochs=10)
trainer.fit(model, train_loader, val_loader)
# Test
trainer.test(model, test_loader)
Automatic features:
# Same code as single GPU!
model = LitModel()
# 8 GPUs with DDP (automatic!)
trainer = L.Trainer(
accelerator='gpu',
devices=8,
strategy='ddp' # Or 'fsdp', 'deepspeed'
)
trainer.fit(model, train_loader)
Launch:
# Single command, Lightning handles the rest
python train.py
No changes needed:
num_nodes=2)from lightning.pytorch.callbacks import ModelCheckpoint, EarlyStopping, LearningRateMonitor
# Create callbacks
checkpoint = ModelCheckpoint(
monitor='val_loss',
mode='min',
save_top_k=3,
filename='model-{epoch:02d}-{val_loss:.2f}'
)
early_stop = EarlyStopping(
monitor='val_loss',
patience=5,
mode='min'
)
lr_monitor = LearningRateMonitor(logging_interval='epoch')
# Add to Trainer
trainer = L.Trainer(
max_epochs=100,
callbacks=[checkpoint, early_stop, lr_monitor]
)
trainer.fit(model, train_loader, val_loader)
Result:
class LitModel(L.LightningModule):
# ... (training_step, etc.)
def configure_optimizers(self):
optimizer = torch.optim.Adam(self.parameters(), lr=1e-3)
# Cosine annealing
scheduler = torch.optim.lr_scheduler.CosineAnnealingLR(
optimizer,
T_max=100,
eta_min=1e-5
)
return {
'optimizer': optimizer,
'lr_scheduler': {
'scheduler': scheduler,
'interval': 'epoch', # Update per epoch
'frequency': 1
}
}
# Learning rate auto-logged!
trainer = L.Trainer(max_epochs=100)
trainer.fit(model, train_loader)
Use PyTorch Lightning when:
Key advantages:
Use alternatives instead:
Issue: Loss not decreasing
Check data and model setup:
# Add to training_step
def training_step(self, batch, batch_idx):
if batch_idx == 0:
print(f"Batch shape: {batch[0].shape}")
print(f"Labels: {batch[1]}")
loss = ...
return loss
Issue: Out of memory
Reduce batch size or use gradient accumulation:
trainer = L.Trainer(
accumulate_grad_batches=4, # Effective batch = batch_size × 4
precision='bf16' # Or 'fp16', reduces memory 50%
)
Issue: Validation not running
Ensure you pass val_loader:
# WRONG
trainer.fit(model, train_loader)
# CORRECT
trainer.fit(model, train_loader, val_loader)
Issue: DDP spawns multiple processes unexpectedly
Lightning auto-detects GPUs. Explicitly set devices:
# Test on CPU first
trainer = L.Trainer(accelerator='cpu', devices=1)
# Then GPU
trainer = L.Trainer(accelerator='gpu', devices=1)
Callbacks: See references/callbacks.md for EarlyStopping, ModelCheckpoint, custom callbacks, and callback hooks.
Distributed strategies: See references/distributed.md for DDP, FSDP, DeepSpeed ZeRO integration, multi-node setup.
Hyperparameter tuning: See references/hyperparameter-tuning.md for integration with Optuna, Ray Tune, and WandB sweeps.
Precision options:
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