testbed/huggingface__accelerate/docs/source/basic_tutorials/migration.mdx

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# Migrating your code to 🤗 Accelerate

This tutorial will detail how to easily convert existing PyTorch code to use 🤗 Accelerate!
You'll see that by just changing a few lines of code, 🤗 Accelerate can perform its magic and get you on 
your way towards running your code on distributed systems with ease!

## The base training loop

To begin, write out a very basic PyTorch training loop. 

<Tip>

    We are under the presumption that `training_dataloader`, `model`, `optimizer`, `scheduler`, and `loss_function` have been defined beforehand.

</Tip>

```python
device = "cuda"
model.to(device)

for batch in training_dataloader:
    optimizer.zero_grad()
    inputs, targets = batch
    inputs = inputs.to(device)
    targets = targets.to(device)
    outputs = model(inputs)
    loss = loss_function(outputs, targets)
    loss.backward()
    optimizer.step()
    scheduler.step()
```

## Add in 🤗 Accelerate

To start using 🤗 Accelerate, first import and create an [`Accelerator`] instance:
```python
from accelerate import Accelerator

accelerator = Accelerator()
```
[`Accelerator`] is the main force behind utilizing all the possible options for distributed training!

### Setting the right device

The [`Accelerator`] class knows the right device to move any PyTorch object to at any time, so you should
change the definition of `device` to come from [`Accelerator`]:

```diff
- device = 'cuda'
+ device = accelerator.device
  model.to(device)
```

### Preparing your objects

Next you need to pass all of the important objects related to training into [`~Accelerator.prepare`]. 🤗 Accelerate will
make sure everything is setup in the current environment for you to start training:

```
model, optimizer, training_dataloader, scheduler = accelerator.prepare(
    model, optimizer, training_dataloader, scheduler
)
```
These objects are returned in the same order they were sent in with. By default when using `device_placement=True`, all of the objects that can be sent to the right device will be.
If you need to work with data that isn't passed to [~Accelerator.prepare] but should be on the active device, you should pass in the `device` you made earlier. 

<Tip warning={true}>

    Accelerate will only prepare objects that inherit from their respective PyTorch classes (such as `torch.optim.Optimizer`).

</Tip>

### Modifying the training loop

Finally, three lines of code need to be changed in the training loop. 🤗 Accelerate's DataLoader classes will automatically handle the device placement by default,
and [`~Accelerator.backward`] should be used for performing the backward pass:

```diff
-   inputs = inputs.to(device)
-   targets = targets.to(device)
    outputs = model(inputs)
    loss = loss_function(outputs, targets)
-   loss.backward()
+   accelerator.backward(loss)
```

With that, your training loop is now ready to use 🤗 Accelerate!

## The finished code

Below is the final version of the converted code: 

```python
from accelerate import Accelerator

accelerator = Accelerator()

model, optimizer, training_dataloader, scheduler = accelerator.prepare(
    model, optimizer, training_dataloader, scheduler
)

for batch in training_dataloader:
    optimizer.zero_grad()
    inputs, targets = batch
    outputs = model(inputs)
    loss = loss_function(outputs, targets)
    accelerator.backward(loss)
    optimizer.step()
    scheduler.step()
```