import tensorflow as tf
import tensorflow_model_optimization as tfmot
import tf_keras as keras

model = keras.Sequential([...])

pruning_schedule = tfmot.sparsity.keras.PolynomialDecay(
                      initial_sparsity=0.0, final_sparsity=0.5,
                      begin_step=2000, end_step=4000)

model_for_pruning = tfmot.sparsity.keras.prune_low_magnitude(
    model, pruning_schedule=pruning_schedule)
...

model_for_pruning.fit(...)