Example: distributed training via PyTorch Lightning

This script passes argparse arguments to PyTorch Lightning Trainer automatically, for example:

$ python examples/svi_lightning.py --accelerator gpu --devices 2 --max_epochs 100 --strategy ddp

View svi_lightning.py on github

# Copyright Contributors to the Pyro project.
# SPDX-License-Identifier: Apache-2.0

# Distributed training via Pytorch Lightning.
#
# This tutorial demonstrates how to distribute SVI training across multiple
# machines (or multiple GPUs on one or more machines) using the PyTorch Lightning
# library. PyTorch Lightning enables data-parallel training by aggregating stochastic
# gradients at each step of training. We focus on integration between PyTorch Lightning and Pyro.
# For further details on distributed computing with PyTorch Lightning, see
#   https://lightning.ai/docs/pytorch/latest
#
# This assumes you have installed pytorch lightning, e.g. via
#   pip install pyro-ppl[lightning]

import argparse

import lightning.pytorch as pl
import torch

import pyro
import pyro.distributions as dist
from pyro.infer import Trace_ELBO
from pyro.infer.autoguide import AutoNormal
from pyro.nn import PyroModule


# We define a model as usual, with no reference to Pytorch Lightning.
# This model is data parallel and supports subsampling.
class Model(PyroModule):
    def __init__(self, size):
        super().__init__()
        self.size = size

    def forward(self, covariates, data=None):
        coeff = pyro.sample("coeff", dist.Normal(0, 1))
        bias = pyro.sample("bias", dist.Normal(0, 1))
        scale = pyro.sample("scale", dist.LogNormal(0, 1))

        # Since we'll use a distributed dataloader during training, we need to
        # manually pass minibatches of (covariates,data) that are smaller than
        # the full self.size. In particular we cannot rely on pyro.plate to
        # automatically subsample, since that would lead to all workers drawing
        # identical subsamples.
        with pyro.plate("data", self.size, len(covariates)):
            loc = bias + coeff * covariates
            return pyro.sample("obs", dist.Normal(loc, scale), obs=data)


# We define an ELBO loss, a PyTorch optimizer, and a training step in our PyroLightningModule.
# Note that we are using a PyTorch optimizer instead of a Pyro optimizer and
# we are using ``training_step`` instead of Pyro's SVI machinery.
class PyroLightningModule(pl.LightningModule):
    def __init__(self, loss_fn: pyro.infer.elbo.ELBOModule, lr: float):
        super().__init__()
        self.loss_fn = loss_fn
        self.model = loss_fn.model
        self.guide = loss_fn.guide
        self.lr = lr
        self.predictive = pyro.infer.Predictive(
            self.model, guide=self.guide, num_samples=1
        )

    def forward(self, *args):
        return self.predictive(*args)

    def training_step(self, batch, batch_idx):
        """Training step for Pyro training."""
        loss = self.loss_fn(*batch)
        # Logging to TensorBoard by default
        self.log("train_loss", loss)
        return loss

    def configure_optimizers(self):
        """Configure an optimizer."""
        return torch.optim.Adam(self.loss_fn.parameters(), lr=self.lr)


def main(args):
    # Create a model, synthetic data, a guide, and a lightning module.
    pyro.set_rng_seed(args.seed)
    pyro.settings.set(module_local_params=True)
    model = Model(args.size)
    covariates = torch.randn(args.size)
    data = model(covariates)
    guide = AutoNormal(model)
    loss_fn = Trace_ELBO()(model, guide)
    training_plan = PyroLightningModule(loss_fn, args.learning_rate)

    # Create a dataloader.
    dataset = torch.utils.data.TensorDataset(covariates, data)
    dataloader = torch.utils.data.DataLoader(dataset, batch_size=args.batch_size)

    # All relevant parameters need to be initialized before ``configure_optimizer`` is called.
    # Since we used AutoNormal guide our parameters have not be initialized yet.
    # Therefore we initialize the model and guide by running one mini-batch through the loss.
    mini_batch = dataset[: args.batch_size]
    loss_fn(*mini_batch)

    # Run stochastic variational inference using PyTorch Lightning Trainer.
    trainer = pl.Trainer(
        accelerator=args.accelerator,
        strategy=args.strategy,
        devices=args.devices,
        max_epochs=args.max_epochs,
    )
    trainer.fit(training_plan, train_dataloaders=dataloader)


if __name__ == "__main__":
    assert pyro.__version__.startswith("1.9.0")
    parser = argparse.ArgumentParser(
        description="Distributed training via PyTorch Lightning"
    )
    parser.add_argument("--size", default=1000000, type=int)
    parser.add_argument("--batch_size", default=100, type=int)
    parser.add_argument("--learning_rate", default=0.01, type=float)
    parser.add_argument("--seed", default=20200723, type=int)
    # pl.Trainer arguments.
    parser.add_argument("--accelerator", default="auto")
    parser.add_argument("--strategy", default="auto")
    parser.add_argument("--devices", default="auto")
    parser.add_argument("--max_epochs", default=None)
    args = parser.parse_args()
    main(args)