[nnx] add data parallel toy example

examples/nnx_toy_examples/04_data_parallel_with_jit.py

@@ -0,0 +1,88 @@
+import os
+os.environ['XLA_FLAGS'] = '--xla_force_host_platform_device_count=8'
+
+import jax
+import jax.numpy as jnp
+import numpy as np
+import optax
+from flax import nnx
+from jax.experimental import mesh_utils
+import matplotlib.pyplot as plt
+
+# create a mesh + shardings
+num_devices = jax.local_device_count()
+mesh = jax.sharding.Mesh(
+  mesh_utils.create_device_mesh((num_devices,)), ('data',)
+)
+model_sharding = jax.NamedSharding(mesh, jax.sharding.PartitionSpec())
+data_sharding = jax.NamedSharding(mesh, jax.sharding.PartitionSpec('data'))
+
+
+# create model
+class MLP(nnx.Module):
+  def __init__(self, din, dmid, dout, *, rngs: nnx.Rngs):
+    self.linear1 = nnx.Linear(din, dmid, rngs=rngs)
+    self.linear2 = nnx.Linear(dmid, dout, rngs=rngs)
+
+  def __call__(self, x):
+    return self.linear2(nnx.relu(self.linear1(x)))
+
+
+model = MLP(1, 64, 1, rngs=nnx.Rngs(0))
+optimizer = nnx.Optimizer(model, optax.adamw(1e-2))
+
+# replicate state
+state = nnx.state((model, optimizer))
+state = jax.device_put(state, model_sharding)
+nnx.update((model, optimizer), state)
+
+# visualize model sharding
+print('model sharding')
+jax.debug.visualize_array_sharding(model.linear1.kernel.value)
+
+
+@nnx.jit
+def train_step(model: MLP, optimizer: nnx.Optimizer, x, y):
+  def loss_fn(model: MLP):
+    y_pred = model(x)
+    return jnp.mean((y - y_pred) ** 2)
+
+  loss, grads = nnx.value_and_grad(loss_fn)(model)
+  optimizer.update(grads)
+  return loss
+
+
+def dataset(steps, batch_size):
+  for _ in range(steps):
+    x = np.random.uniform(-2, 2, size=(batch_size, 1))
+    y = 0.8 * x**2 + 0.1 + np.random.normal(0, 0.1, size=x.shape)
+    yield x, y
+
+
+for step, (x, y) in enumerate(dataset(1000, 16)):
+  # shard data
+  x, y = jax.device_put((x, y), data_sharding)
+  # train
+  loss = train_step(model, optimizer, x, y)
+
+  if step == 0:
+    print('data sharding')
+    jax.debug.visualize_array_sharding(x)
+
+  if step % 100 == 0:
+    print(f'step={step}, loss={loss}')
+
+# dereplicate state
+state = nnx.state((model, optimizer))
+state = jax.device_get(state)
+nnx.update((model, optimizer), state)
+
+X, Y = next(dataset(1, 1000))
+x_range = np.linspace(X.min(), X.max(), 100)[:, None]
+y_pred = model(x_range)
+
+# plot
+plt.scatter(X, Y, label='data')
+plt.plot(x_range, y_pred, color='black', label='model')
+plt.legend()
+plt.show()