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# Checkpointing

When training a PyTorch model with Accelerate, you may often want to save and continue a state of training. Doing so requires
saving and loading the model, optimizer, RNG generators, and the GradScaler. Inside Accelerate are two convenience functions to achieve this quickly:
- Use [`~Accelerator.save_state`] for saving everything mentioned above to a folder location
- Use [`~Accelerator.load_state`] for loading everything stored from an earlier `save_state`

To further customize where and how states are saved through [`~Accelerator.save_state`] the [`~utils.ProjectConfiguration`] class can be used. For example 
if `automatic_checkpoint_naming` is enabled each saved checkpoint will be located then at `Accelerator.project_dir/checkpoints/checkpoint_{checkpoint_number}`.

It should be noted that the expectation is that those states come from the same training script, they should not be from two separate scripts.

- By using [`~Accelerator.register_for_checkpointing`], you can register custom objects to be automatically stored or loaded from the two prior functions,
so long as the object has a `state_dict` **and** a `load_state_dict` functionality. This could include objects such as a learning rate scheduler. 


Below is a brief example using checkpointing to save and reload a state during training:

```python
from accelerate import Accelerator
import torch

accelerator = Accelerator(project_dir="my/save/path")

my_scheduler = torch.optim.lr_scheduler.StepLR(my_optimizer, step_size=1, gamma=0.99)
my_model, my_optimizer, my_training_dataloader = accelerator.prepare(my_model, my_optimizer, my_training_dataloader)

# Register the LR scheduler
accelerator.register_for_checkpointing(my_scheduler)

# Save the starting state
accelerator.save_state()

device = accelerator.device
my_model.to(device)

# Perform training
for epoch in range(num_epochs):
    for batch in my_training_dataloader:
        my_optimizer.zero_grad()
        inputs, targets = batch
        inputs = inputs.to(device)
        targets = targets.to(device)
        outputs = my_model(inputs)
        loss = my_loss_function(outputs, targets)
        accelerator.backward(loss)
        my_optimizer.step()
    my_scheduler.step()

# Restore the previous state
accelerator.load_state("my/save/path/checkpointing/checkpoint_0")
```

## Restoring the state of the DataLoader 

After resuming from a checkpoint, it may also be desirable to resume from a particular point in the active `DataLoader` if 
the state was saved during the middle of an epoch. You can use [`~Accelerator.skip_first_batches`] to do so. 

```python
from accelerate import Accelerator

accelerator = Accelerator(project_dir="my/save/path")

train_dataloader = accelerator.prepare(train_dataloader)
accelerator.load_state("my_state")

# Assume the checkpoint was saved 100 steps into the epoch
skipped_dataloader = accelerator.skip_first_batches(train_dataloader, 100)

# After the first iteration, go back to `train_dataloader`

# First epoch
for batch in skipped_dataloader:
    # Do something
    pass

# Second epoch
for batch in train_dataloader:
    # Do something
    pass
```
