
Distributed training with 🤗 Accelerate
As models get bigger, parallelism has emerged as a strategy for training larger models on limited hardware and accelerating training speed by several orders of magnitude. At Hugging Face, we created the 🤗 Accelerate library to help users easily train a 🤗 Transformers model on any type of distributed setup, whether it is multiple GPU's on one machine or multiple GPU's across several machines. In this tutorial, learn how to customize your native PyTorch training loop to enable training in a distributed environment.
Setup
Get started by installing 🤗 Accelerate:

pip install accelerate
Then import and create an [~accelerate.Accelerator] object. The [~accelerate.Accelerator] will automatically detect your type of distributed setup and initialize all the necessary components for training. You don't need to explicitly place your model on a device.

from accelerate import Accelerator
accelerator = Accelerator()

Prepare to accelerate
The next step is to pass all the relevant training objects to the [~accelerate.Accelerator.prepare] method. This includes your training and evaluation DataLoaders, a model and an optimizer:

train_dataloader, eval_dataloader, model, optimizer = accelerator.prepare(
     train_dataloader, eval_dataloader, model, optimizer
 )

Backward
The last addition is to replace the typical loss.backward() in your training loop with 🤗 Accelerate's [~accelerate.Accelerator.backward]method:

for epoch in range(num_epochs):
     for batch in train_dataloader:
         outputs = model(**batch)
         loss = outputs.loss
         accelerator.backward(loss)

         optimizer.step()
         lr_scheduler.step()
         optimizer.zero_grad()
         progress_bar.update(1)

As you can see in the following code, you only need to add four additional lines of code to your training loop to enable distributed training!

+ from accelerate import Accelerator
  from transformers import AdamW, AutoModelForSequenceClassification, get_scheduler

accelerator = Accelerator()

model = AutoModelForSequenceClassification.from_pretrained(checkpoint, num_labels=2)
  optimizer = AdamW(model.parameters(), lr=3e-5)

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

model.to(device)

train_dataloader, eval_dataloader, model, optimizer = accelerator.prepare(

train_dataloader, eval_dataloader, model, optimizer
)

num_epochs = 3
  num_training_steps = num_epochs * len(train_dataloader)
  lr_scheduler = get_scheduler(
      "linear",
      optimizer=optimizer,
      num_warmup_steps=0,
      num_training_steps=num_training_steps
  )
progress_bar = tqdm(range(num_training_steps))
model.train()
  for epoch in range(num_epochs):
      for batch in train_dataloader:

          outputs = model(**batch)
          loss = outputs.loss

+         accelerator.backward(loss)
      optimizer.step()
      lr_scheduler.step()
      optimizer.zero_grad()
      progress_bar.update(1)

Train
Once you've added the relevant lines of code, launch your training in a script or a notebook like Colaboratory.
Train with a script
If you are running your training from a script, run the following command to create and save a configuration file:

accelerate config
Then launch your training with:

accelerate launch train.py
Train with a notebook
🤗 Accelerate can also run in a notebook if you're planning on using Colaboratory's TPUs. Wrap all the code responsible for training in a function, and pass it to [~accelerate.notebook_launcher]:

from accelerate import notebook_launcher
notebook_launcher(training_function)

For more information about 🤗 Accelerate and it's rich features, refer to the documentation.