Keras Transfer Learning

# Credits: 
#   Author: Gabriel Cassimiro 
#   Blog post: https://towardsdatascience.com/transfer-learning-with-vgg16-and-keras-50ea161580b4
#   GitHub Repo: https://github.com/gabrielcassimiro17/object-detection
#
import tensorflow_datasets as tfds
from tensorflow.keras import layers, models
from tensorflow.keras.utils import to_categorical
from tensorflow.keras.callbacks import EarlyStopping

## Loading images and labels
(train_ds, train_labels), (test_ds, test_labels) = tfds.load(
    "tf_flowers",
    split=["train[:70%]", "train[:30%]"], ## Train test split
    batch_size=-1,
    as_supervised=True,  # Include labels
)

## Resizing images
train_ds = tf.image.resize(train_ds, (150, 150))
test_ds = tf.image.resize(test_ds, (150, 150))

## Transforming labels to correct format
train_labels = to_categorical(train_labels, num_classes=5)
test_labels = to_categorical(test_labels, num_classes=5)

from tensorflow.keras.applications.vgg16 import VGG16
from tensorflow.keras.applications.vgg16 import preprocess_input

## Loading VGG16 model
base_model = VGG16(weights="imagenet", include_top=False, input_shape=train_ds[0].shape)
base_model.trainable = False ## Not trainable weights

## Preprocessing input
train_ds = preprocess_input(train_ds) 
test_ds = preprocess_input(test_ds)

flatten_layer = layers.Flatten()
dense_layer_1 = layers.Dense(50, activation='relu')
dense_layer_2 = layers.Dense(20, activation='relu')
prediction_layer = layers.Dense(5, activation='softmax')

model = models.Sequential([
    base_model,
    flatten_layer,
    dense_layer_1,
    dense_layer_2,
    prediction_layer
])

model.compile(
    optimizer='adam',
    loss='categorical_crossentropy',
    metrics=['accuracy'],
)

es = EarlyStopping(monitor='val_accuracy', mode='max', patience=5,  restore_best_weights=True)

model.fit(train_ds, train_labels, epochs=50, validation_split=0.2, batch_size=32, callbacks=[es])

model.evaluate(test_ds, test_labels)