知识蒸馏技巧

!pip install -q tensorflow-addons

from tensorflow import keras
import tensorflow_addons as tfa
import tensorflow as tf

import matplotlib.pyplot as plt
import numpy as np

import tensorflow_datasets as tfds

tfds.disable_progress_bar()

AUTO = tf.data.AUTOTUNE  # Used to dynamically adjust parallelism.
BATCH_SIZE = 64

# Comes from Table 4 and "Training setup" section.
TEMPERATURE = 10  # Used to soften the logits before they go to softmax.
INIT_LR = 0.003  # Initial learning rate that will be decayed over the training period.
WEIGHT_DECAY = 0.001  # Used for regularization.
CLIP_THRESHOLD = 1.0  # Used for clipping the gradients by L2-norm.

# We will first resize the training images to a bigger size and then we will take
# random crops of a lower size.
BIGGER = 160
RESIZE = 128

train_ds, validation_ds, test_ds = tfds.load(
    "oxford_flowers102", split=["train", "validation", "test"], as_supervised=True
)
print(f"Number of training examples: {train_ds.cardinality()}.")
print(
    f"Number of validation examples: {validation_ds.cardinality()}."
)
print(f"Number of test examples: {test_ds.cardinality()}.")

import os

os.environ["KAGGLE_USERNAME"] = "" # TODO: enter your Kaggle user name here
os.environ["KAGGLE_KEY"] = "" # TODO: enter your Kaggle key here

$ kaggle datasets download -d spsayakpaul/bitresnet101x3flowers102
$ unzip -qq bitresnet101x3flowers102.zip

import os

os.environ["KAGGLE_USERNAME"] = ""  # TODO: enter your Kaggle user name here
os.environ["KAGGLE_KEY"] = ""  # TODO: enter your Kaggle API key here

!kaggle datasets download -d spsayakpaul/bitresnet101x3flowers102

!unzip -qq bitresnet101x3flowers102.zip

# Since the teacher model is not going to be trained further we make
# it non-trainable.
teacher_model = keras.models.load_model(
    "/home/jupyter/keras-io/examples/keras_recipes/T-r101x3-128"
)
teacher_model.trainable = False
teacher_model.summary()

def mixup(images, labels):
    alpha = tf.random.uniform([], 0, 1)
    mixedup_images = alpha * images + (1 - alpha) * tf.reverse(images, axis=[0])
    # The labels do not matter here since they are NOT used during
    # training.
    return mixedup_images, labels


def preprocess_image(image, label, train=True):
    image = tf.cast(image, tf.float32) / 255.0

    if train:
        image = tf.image.resize(image, (BIGGER, BIGGER))
        image = tf.image.random_crop(image, (RESIZE, RESIZE, 3))
        image = tf.image.random_flip_left_right(image)
    else:
        # Central fraction amount is from here:
        # https://git.io/J8Kda.
        image = tf.image.central_crop(image, central_fraction=0.875)
        image = tf.image.resize(image, (RESIZE, RESIZE))

    return image, label


def prepare_dataset(dataset, train=True, batch_size=BATCH_SIZE):
    if train:
        dataset = dataset.map(preprocess_image, num_parallel_calls=AUTO)
        dataset = dataset.shuffle(BATCH_SIZE * 10)
    else:
        dataset = dataset.map(
            lambda x, y: (preprocess_image(x, y, train)), num_parallel_calls=AUTO
        )
    dataset = dataset.batch(batch_size)

    if train:
        dataset = dataset.map(mixup, num_parallel_calls=AUTO)

    dataset = dataset.prefetch(AUTO)
    return dataset

train_ds = prepare_dataset(train_ds, True)
validation_ds = prepare_dataset(validation_ds, False)
test_ds = prepare_dataset(test_ds, False)

sample_images, _ = next(iter(train_ds))
plt.figure(figsize=(10, 10))
for n in range(25):
    ax = plt.subplot(5, 5, n + 1)
    plt.imshow(sample_images[n].numpy())
    plt.axis("off")
plt.show()

def get_resnetv2():
    resnet_v2 = keras.applications.ResNet50V2(
        weights=None,
        input_shape=(RESIZE, RESIZE, 3),
        classes=102,
        classifier_activation="linear",
    )
    return resnet_v2


get_resnetv2().count_params()

class Distiller(tf.keras.Model):
    def __init__(self, student, teacher):
        super().__init__()
        self.student = student
        self.teacher = teacher
        self.loss_tracker = keras.metrics.Mean(name="distillation_loss")

    @property
    def metrics(self):
        metrics = super().metrics
        metrics.append(self.loss_tracker)
        return metrics

    def compile(
        self, optimizer, metrics, distillation_loss_fn, temperature=TEMPERATURE,
    ):
        super().compile(optimizer=optimizer, metrics=metrics)
        self.distillation_loss_fn = distillation_loss_fn
        self.temperature = temperature

    def train_step(self, data):
        # Unpack data
        x, _ = data

        # Forward pass of teacher
        teacher_predictions = self.teacher(x, training=False)

        with tf.GradientTape() as tape:
            # Forward pass of student
            student_predictions = self.student(x, training=True)

            # Compute loss
            distillation_loss = self.distillation_loss_fn(
                tf.nn.softmax(teacher_predictions / self.temperature, axis=1),
                tf.nn.softmax(student_predictions / self.temperature, axis=1),
            )

        # Compute gradients
        trainable_vars = self.student.trainable_variables
        gradients = tape.gradient(distillation_loss, trainable_vars)

        # Update weights
        self.optimizer.apply_gradients(zip(gradients, trainable_vars))

        # Report progress
        self.loss_tracker.update_state(distillation_loss)
        return {"distillation_loss": self.loss_tracker.result()}

    def test_step(self, data):
        # Unpack data
        x, y = data

        # Forward passes
        teacher_predictions = self.teacher(x, training=False)
        student_predictions = self.student(x, training=False)

        # Calculate the loss
        distillation_loss = self.distillation_loss_fn(
            tf.nn.softmax(teacher_predictions / self.temperature, axis=1),
            tf.nn.softmax(student_predictions / self.temperature, axis=1),
        )

        # Report progress
        self.loss_tracker.update_state(distillation_loss)
        self.compiled_metrics.update_state(y, student_predictions)
        results = {m.name: m.result() for m in self.metrics}
        return results

# Some code is taken from:
# https://www.kaggle.com/ashusma/training-rfcx-tensorflow-tpu-effnet-b2.


class WarmUpCosine(keras.optimizers.schedules.LearningRateSchedule):
    def __init__(
        self, learning_rate_base, total_steps, warmup_learning_rate, warmup_steps
    ):
        super().__init__()

        self.learning_rate_base = learning_rate_base
        self.total_steps = total_steps
        self.warmup_learning_rate = warmup_learning_rate
        self.warmup_steps = warmup_steps
        self.pi = tf.constant(np.pi)

    def __call__(self, step):
        if self.total_steps < self.warmup_steps:
            raise ValueError("Total_steps must be larger or equal to warmup_steps.")

        cos_annealed_lr = tf.cos(
            self.pi
            * (tf.cast(step, tf.float32) - self.warmup_steps)
            / float(self.total_steps - self.warmup_steps)
        )
        learning_rate = 0.5 * self.learning_rate_base * (1 + cos_annealed_lr)

        if self.warmup_steps > 0:
            if self.learning_rate_base < self.warmup_learning_rate:
                raise ValueError(
                    "Learning_rate_base must be larger or equal to "
                    "warmup_learning_rate."
                )
            slope = (
                self.learning_rate_base - self.warmup_learning_rate
            ) / self.warmup_steps
            warmup_rate = slope * tf.cast(step, tf.float32) + self.warmup_learning_rate
            learning_rate = tf.where(
                step < self.warmup_steps, warmup_rate, learning_rate
            )
        return tf.where(
            step > self.total_steps, 0.0, learning_rate, name="learning_rate"
        )

ARTIFICIAL_EPOCHS = 1000
ARTIFICIAL_BATCH_SIZE = 512
DATASET_NUM_TRAIN_EXAMPLES = 1020
TOTAL_STEPS = int(
    DATASET_NUM_TRAIN_EXAMPLES / ARTIFICIAL_BATCH_SIZE * ARTIFICIAL_EPOCHS
)
scheduled_lrs = WarmUpCosine(
    learning_rate_base=INIT_LR,
    total_steps=TOTAL_STEPS,
    warmup_learning_rate=0.0,
    warmup_steps=1500,
)

lrs = [scheduled_lrs(step) for step in range(TOTAL_STEPS)]
plt.plot(lrs)
plt.xlabel("Step", fontsize=14)
plt.ylabel("LR", fontsize=14)
plt.show()

optimizer = tfa.optimizers.AdamW(
    weight_decay=WEIGHT_DECAY, learning_rate=scheduled_lrs, clipnorm=CLIP_THRESHOLD
)

student_model = get_resnetv2()

distiller = Distiller(student=student_model, teacher=teacher_model)
distiller.compile(
    optimizer,
    metrics=[keras.metrics.SparseCategoricalAccuracy()],
    distillation_loss_fn=keras.losses.KLDivergence(),
    temperature=TEMPERATURE,
)

history = distiller.fit(
    train_ds,
    steps_per_epoch=int(np.ceil(DATASET_NUM_TRAIN_EXAMPLES / BATCH_SIZE)),
    validation_data=validation_ds,
    epochs=30,  # This should be at least 1000.
)

student = distiller.student
student_model.compile(metrics=["accuracy"])
_, top1_accuracy = student.evaluate(test_ds)
print(f"Top-1 accuracy on the test set: {round(top1_accuracy * 100, 2)}%")

# Download the pre-trained weights.
!wget https://git.io/JBO3Y -O S-r50x1-128-1000.tar.gz
!tar xf S-r50x1-128-1000.tar.gz

pretrained_student = keras.models.load_model("S-r50x1-128-1000")
pretrained_student.summary()

_, top1_accuracy = pretrained_student.evaluate(test_ds)
print(f"Top-1 accuracy on the test set: {round(top1_accuracy * 100, 2)}%")