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Given the following text description, write Python code to implement the functionality described below step by step
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Description:
 Copyright 2019 The TensorFlow Authors.
Step1:  Post-training dynamic range quantization
<table class="tfo-notebook-buttons" align="left">
  <td>
    <a target="_blank" href="https
Step2:  Train a TensorFlow model
Step3:  For the example, since you trained the model for just a single epoch, so it only trains to ~96% accuracy.
Convert to a TensorFlow Lite model
Using the Python TFLiteConverter, you can now convert the trained model into a TensorFlow Lite model.
Now load the model using the TFLiteConverter
Step4:  Write it out to a tflite file
Step5:  To quantize the model on export, set the optimizations flag to optimize for size
Step6:  Note how the resulting file, is approximately 1/4 the size.
Step7:  Run the TFLite models
Run the TensorFlow Lite model using the Python TensorFlow Lite
Interpreter.
Load the model into an interpreter
Step8:  Test the model on one image
Step9:  Evaluate the models
Step10:  Repeat the evaluation on the dynamic range quantized model to obtain
Step11:  In this example, the compressed model has no difference in the accuracy.
Optimizing an existing model
Resnets with pre-activation layers (Resnet-v2) are widely used for vision applications.
  Pre-trained frozen graph for resnet-v2-101 is available on
  Tensorflow Hub.
You can convert the frozen graph to a TensorFLow Lite flatbuffer with quantization by
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Python Code:
#@title 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.
Explanation: Copyright 2019 The TensorFlow Authors.
End of explanation
import logging
logging.getLogger("tensorflow").setLevel(logging.DEBUG)
import tensorflow as tf
from tensorflow import keras
import numpy as np
import pathlib
Explanation: Post-training dynamic range quantization
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    <a target="_blank" href="https://www.tensorflow.org/lite/performance/post_training_quant"><img src="https://www.tensorflow.org/images/tf_logo_32px.png" />View on TensorFlow.org</a>
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  <td>
    <a target="_blank" href="https://colab.research.google.com/github/tensorflow/tensorflow/blob/master/tensorflow/lite/g3doc/performance/post_training_quant.ipynb"><img src="https://www.tensorflow.org/images/colab_logo_32px.png" />Run in Google Colab</a>
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  <td>
    <a target="_blank" href="https://github.com/tensorflow/tensorflow/blob/master/tensorflow/lite/g3doc/performance/post_training_quant.ipynb"><img src="https://www.tensorflow.org/images/GitHub-Mark-32px.png" />View source on GitHub</a>
  </td>
  <td>
    <a href="https://storage.googleapis.com/tensorflow_docs/tensorflow/tensorflow/lite/g3doc/performance/post_training_quant.ipynb"><img src="https://www.tensorflow.org/images/download_logo_32px.png" />Download notebook</a>
  </td>
  <td>
    <a href="https://tfhub.dev/google/imagenet/resnet_v2_101/classification/4"><img src="https://www.tensorflow.org/images/hub_logo_32px.png" />See TF Hub model</a>
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Overview
TensorFlow Lite now supports
converting weights to 8 bit precision as part of model conversion from
tensorflow graphdefs to TensorFlow Lite's flat buffer format. Dynamic range quantization achieves a 4x reduction in the model size. In addition, TFLite supports on the fly quantization and dequantization of activations to allow for:
Using quantized kernels for faster implementation when available.
Mixing of floating-point kernels with quantized kernels for different parts
    of the graph.
The activations are always stored in floating point. For ops that
support quantized kernels, the activations are quantized to 8 bits of precision
dynamically prior to processing and are de-quantized to float precision after
processing. Depending on the model being converted, this can give a speedup over
pure floating point computation.
In contrast to
quantization aware training
, the weights are quantized post training and the activations are quantized dynamically 
at inference in this method.
Therefore, the model weights are not retrained to compensate for quantization
induced errors. It is important to check the accuracy of the quantized model to
ensure that the degradation is acceptable.
This tutorial trains an MNIST model from scratch, checks its accuracy in
TensorFlow, and then converts the model into a Tensorflow Lite flatbuffer
with dynamic range quantization. Finally, it checks the
accuracy of the converted model and compare it to the original float model.
Build an MNIST model
Setup
End of explanation
# Load MNIST dataset
mnist = keras.datasets.mnist
(train_images, train_labels), (test_images, test_labels) = mnist.load_data()
# Normalize the input image so that each pixel value is between 0 to 1.
train_images = train_images / 255.0
test_images = test_images / 255.0
# Define the model architecture
model = keras.Sequential([
  keras.layers.InputLayer(input_shape=(28, 28)),
  keras.layers.Reshape(target_shape=(28, 28, 1)),
  keras.layers.Conv2D(filters=12, kernel_size=(3, 3), activation=tf.nn.relu),
  keras.layers.MaxPooling2D(pool_size=(2, 2)),
  keras.layers.Flatten(),
  keras.layers.Dense(10)
])
# Train the digit classification model
model.compile(optimizer='adam',
              loss=keras.losses.SparseCategoricalCrossentropy(from_logits=True),
              metrics=['accuracy'])
model.fit(
  train_images,
  train_labels,
  epochs=1,
  validation_data=(test_images, test_labels)
)
Explanation: Train a TensorFlow model
End of explanation
converter = tf.lite.TFLiteConverter.from_keras_model(model)
tflite_model = converter.convert()
Explanation: For the example, since you trained the model for just a single epoch, so it only trains to ~96% accuracy.
Convert to a TensorFlow Lite model
Using the Python TFLiteConverter, you can now convert the trained model into a TensorFlow Lite model.
Now load the model using the TFLiteConverter:
End of explanation
tflite_models_dir = pathlib.Path("/tmp/mnist_tflite_models/")
tflite_models_dir.mkdir(exist_ok=True, parents=True)
tflite_model_file = tflite_models_dir/"mnist_model.tflite"
tflite_model_file.write_bytes(tflite_model)
Explanation: Write it out to a tflite file:
End of explanation
converter.optimizations = [tf.lite.Optimize.DEFAULT]
tflite_quant_model = converter.convert()
tflite_model_quant_file = tflite_models_dir/"mnist_model_quant.tflite"
tflite_model_quant_file.write_bytes(tflite_quant_model)
Explanation: To quantize the model on export, set the optimizations flag to optimize for size:
End of explanation
!ls -lh {tflite_models_dir}
Explanation: Note how the resulting file, is approximately 1/4 the size.
End of explanation
interpreter = tf.lite.Interpreter(model_path=str(tflite_model_file))
interpreter.allocate_tensors()
interpreter_quant = tf.lite.Interpreter(model_path=str(tflite_model_quant_file))
interpreter_quant.allocate_tensors()
Explanation: Run the TFLite models
Run the TensorFlow Lite model using the Python TensorFlow Lite
Interpreter.
Load the model into an interpreter
End of explanation
test_image = np.expand_dims(test_images[0], axis=0).astype(np.float32)
input_index = interpreter.get_input_details()[0]["index"]
output_index = interpreter.get_output_details()[0]["index"]
interpreter.set_tensor(input_index, test_image)
interpreter.invoke()
predictions = interpreter.get_tensor(output_index)
import matplotlib.pylab as plt
plt.imshow(test_images[0])
template = "True:{true}, predicted:{predict}"
_ = plt.title(template.format(true= str(test_labels[0]),
                              predict=str(np.argmax(predictions[0]))))
plt.grid(False)
Explanation: Test the model on one image
End of explanation
# A helper function to evaluate the TF Lite model using "test" dataset.
def evaluate_model(interpreter):
  input_index = interpreter.get_input_details()[0]["index"]
  output_index = interpreter.get_output_details()[0]["index"]
  # Run predictions on every image in the "test" dataset.
  prediction_digits = []
  for test_image in test_images:
    # Pre-processing: add batch dimension and convert to float32 to match with
    # the model's input data format.
    test_image = np.expand_dims(test_image, axis=0).astype(np.float32)
    interpreter.set_tensor(input_index, test_image)
    # Run inference.
    interpreter.invoke()
    # Post-processing: remove batch dimension and find the digit with highest
    # probability.
    output = interpreter.tensor(output_index)
    digit = np.argmax(output()[0])
    prediction_digits.append(digit)
  # Compare prediction results with ground truth labels to calculate accuracy.
  accurate_count = 0
  for index in range(len(prediction_digits)):
    if prediction_digits[index] == test_labels[index]:
      accurate_count += 1
  accuracy = accurate_count * 1.0 / len(prediction_digits)
  return accuracy
print(evaluate_model(interpreter))
Explanation: Evaluate the models
End of explanation
print(evaluate_model(interpreter_quant))
Explanation: Repeat the evaluation on the dynamic range quantized model to obtain:
End of explanation
import tensorflow_hub as hub
resnet_v2_101 = tf.keras.Sequential([
  keras.layers.InputLayer(input_shape=(224, 224, 3)),
  hub.KerasLayer("https://tfhub.dev/google/imagenet/resnet_v2_101/classification/4")
])
converter = tf.lite.TFLiteConverter.from_keras_model(resnet_v2_101)
# Convert to TF Lite without quantization
resnet_tflite_file = tflite_models_dir/"resnet_v2_101.tflite"
resnet_tflite_file.write_bytes(converter.convert())
# Convert to TF Lite with quantization
converter.optimizations = [tf.lite.Optimize.DEFAULT]
resnet_quantized_tflite_file = tflite_models_dir/"resnet_v2_101_quantized.tflite"
resnet_quantized_tflite_file.write_bytes(converter.convert())
!ls -lh {tflite_models_dir}/*.tflite
Explanation: In this example, the compressed model has no difference in the accuracy.
Optimizing an existing model
Resnets with pre-activation layers (Resnet-v2) are widely used for vision applications.
  Pre-trained frozen graph for resnet-v2-101 is available on
  Tensorflow Hub.
You can convert the frozen graph to a TensorFLow Lite flatbuffer with quantization by:
End of explanation
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