tfp.sts.build_factored_surrogate_posterior

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Build a variational posterior that factors over model parameters.

Used in the notebooks

Used in the tutorials

The surrogate posterior consists of independent Normal distributions for each parameter with trainable loc and scale, transformed using the parameter's bijector to the appropriate support space for that parameter.

model An instance of StructuralTimeSeries representing a time-series model. This represents a joint distribution over time-series and their parameters with batch shape [b1, ..., bN].
batch_shape Batch shape (Python tuple, list, or int) of initial states to optimize in parallel. Default value: (). (i.e., just run a single optimization).
seed Python integer to seed the random number generator.
name Python str name prefixed to ops created by this function. Default value: None (i.e., 'build_factored_surrogate_posterior').

variational_posterior tfd.JointDistributionNamed defining a trainable surrogate posterior over model parameters. Samples from this distribution are Python dicts with Python str parameter names as keys.

Examples

Assume we've built a structural time-series model:

  day_of_week = tfp.sts.Seasonal(
      num_seasons=7,
      observed_time_series=observed_time_series,
      name='day_of_week')
  local_linear_trend = tfp.sts.LocalLinearTrend(
      observed_time_series=observed_time_series,
      name='local_linear_trend')
  model = tfp.sts.Sum(components=[day_of_week, local_linear_trend],
                      observed_time_series=observed_time_series)

To fit the model to data, we define a surrogate posterior and fit it by optimizing a variational bound:

  surrogate_posterior = tfp.sts.build_factored_surrogate_posterior(
    model=model)
  loss_curve = tfp.vi.fit_surrogate_posterior(
    target_log_prob_fn=model.joint_log_prob(observed_time_series),
    surrogate_posterior=surrogate_posterior,
    optimizer=tf.optimizers.Adam(learning_rate=0.1),
    num_steps=200)
  posterior_samples = surrogate_posterior.sample(50)

  # In graph mode, we would need to write:
  # with tf.control_dependencies([loss_curve]):
  #   posterior_samples = surrogate_posterior.sample(50)

For more control, we can also build and optimize a variational loss manually:

  @tf.function(autograph=False)  # Ensure the loss is computed efficiently
  def loss_fn():
    return tfp.vi.monte_carlo_variational_loss(
      model.joint_log_prob(observed_time_series),
      surrogate_posterior,
      sample_size=10)

  optimizer = tf.optimizers.Adam(learning_rate=0.1)
  for step in range(200):
    with tf.GradientTape() as tape:
      loss = loss_fn()
    grads = tape.gradient(loss, surrogate_posterior.trainable_variables)
    optimizer.apply_gradients(
      zip(grads, surrogate_posterior.trainable_variables))
    if step % 20 == 0:
      print('step {} loss {}'.format(step, loss))

  posterior_samples = surrogate_posterior.sample(50)