mnist_classifier_from_scratch.py

# Copyright 2018 The JAX Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# Modified by Graphcore Ltd 2024.

"""A basic MNIST example using Numpy and JAX.

The primary aim here is simplicity and minimal dependencies.
"""


import time

import datasets
import jax
import jax.numpy as jnp
import numpy as np
import numpy.random as npr
from jax import grad, jit, lax

import jax_scalify as jsa

# from jax.scipy.special import logsumexp


def logsumexp(a, axis=None, keepdims=False):
    dims = (axis,)
    amax = jnp.max(a, axis=dims, keepdims=keepdims)
    # FIXME: not proper scale propagation, introducing NaNs
    # amax = lax.stop_gradient(lax.select(jnp.isfinite(amax), amax, lax.full_like(amax, 0)))
    amax = lax.stop_gradient(amax)
    out = lax.sub(a, amax)
    out = lax.exp(out)
    out = lax.add(lax.log(jnp.sum(out, axis=dims, keepdims=keepdims)), amax)
    return out


def init_random_params(scale, layer_sizes, rng=npr.RandomState(0)):
    return [(scale * rng.randn(m, n), scale * rng.randn(n)) for m, n, in zip(layer_sizes[:-1], layer_sizes[1:])]


def predict(params, inputs):
    activations = inputs
    for w, b in params[:-1]:
        # Matmul + relu
        outputs = jnp.dot(activations, w) + b
        activations = jax.nn.relu(outputs)

    final_w, final_b = params[-1]
    logits = jnp.dot(activations, final_w) + final_b
    # Dynamic rescaling of the gradient, as logits gradient not properly scaled.
    # logits = jsa.ops.dynamic_rescale_l2_grad(logits)
    logits = logits - logsumexp(logits, axis=1, keepdims=True)
    return logits


def loss(params, batch):
    inputs, targets = batch
    preds = predict(params, inputs)
    targets = jsa.lax.rebalance(targets, np.float32(1 / 8))
    return -jnp.mean(jnp.sum(preds * targets, axis=1))


def accuracy(params, batch):
    inputs, targets = batch
    target_class = jnp.argmax(targets, axis=1)
    predicted_class = jnp.argmax(predict(params, inputs), axis=1)
    return jnp.mean(predicted_class == target_class)


if __name__ == "__main__":
    layer_sizes = [784, 512, 512, 10]
    param_scale = 0.1
    step_size = 0.1
    num_epochs = 10
    batch_size = 128

    training_dtype = np.float16
    scale_dtype = np.float32

    train_images, train_labels, test_images, test_labels = datasets.mnist()
    num_train = train_images.shape[0]
    num_complete_batches, leftover = divmod(num_train, batch_size)
    num_batches = num_complete_batches + bool(leftover)

    def data_stream():
        rng = npr.RandomState(0)
        while True:
            perm = rng.permutation(num_train)
            for i in range(num_batches):
                batch_idx = perm[i * batch_size : (i + 1) * batch_size]
                yield train_images[batch_idx], train_labels[batch_idx]

    batches = data_stream()
    params = init_random_params(param_scale, layer_sizes)
    # Transform parameters to `ScaledArray` and proper dtype.
    params = jsa.as_scaled_array(params, scale=scale_dtype(param_scale))
    params = jsa.tree.astype(params, training_dtype)

    @jit
    @jsa.scalify
    def update(params, batch):
        grads = grad(loss)(params, batch)
        return [(w - step_size * dw, b - step_size * db) for (w, b), (dw, db) in zip(params, grads)]

    for epoch in range(num_epochs):
        start_time = time.time()
        for _ in range(num_batches):
            batch = next(batches)
            # Scaled micro-batch + training dtype cast.
            batch = jsa.as_scaled_array(batch, scale=scale_dtype(1))
            batch = jsa.tree.astype(batch, training_dtype)

            with jsa.ScalifyConfig(rounding_mode=jsa.Pow2RoundMode.DOWN, scale_dtype=scale_dtype):
                params = update(params, batch)

        epoch_time = time.time() - start_time

        # Evaluation in normal/unscaled float32, for consistency.
        raw_params = jsa.asarray(params, dtype=np.float32)
        train_acc = accuracy(raw_params, (train_images, train_labels))
        test_acc = accuracy(raw_params, (test_images, test_labels))
        print(f"Epoch {epoch} in {epoch_time:0.2f} sec")
        print(f"Training set accuracy {train_acc:0.5f}")
        print(f"Test set accuracy {test_acc:0.5f}")