tfp.experimental.mcmc.GradientBasedTrajectoryLengthAdaptation

Use gradient ascent to adapt inner kernel's trajectory length.

Inherits From: TransitionKernel

tfp.experimental.mcmc.GradientBasedTrajectoryLengthAdaptation(
    inner_kernel, num_adaptation_steps, use_halton_sequence_jitter=True,
    adaptation_rate=0.025, jitter_amount=1.0,
    criterion_fn=tfp.experimental.mcmc.chees_criterion, max_leapfrog_steps=1000,
    num_leapfrog_steps_getter_fn=hmc_like_num_leapfrog_steps_getter_fn,
    num_leapfrog_steps_setter_fn=hmc_like_num_leapfrog_steps_setter_fn,
    step_size_getter_fn=hmc_like_step_size_getter_fn,
    proposed_velocity_getter_fn=hmc_like_proposed_velocity_getter_fn,
    log_accept_prob_getter_fn=hmc_like_log_accept_prob_getter_fn,
    proposed_state_getter_fn=hmc_like_proposed_state_getter_fn, validate_args=False,
    experimental_shard_axis_names=None, name=None
)

This kernel optimizes the continuous trajectory length (aka integration time) parameter of Hamiltonian Monte Carlo. It does so by following the gradient of a criterion with respect to the trajectory length. The criterion is computed via criterion_fn with signature (previous_state, proposed_state, accept_prob) -> criterion, where both the returned values retain the batch dimensions implied by the first three inputs. See chees_criterion for an example.

To avoid resonances, this kernel jitters the integration time between 0 and the learned trajectory length by default.

The initial trajectory length is extracted from the inner HamiltonianMonteCarlo kernel by multiplying the initial step size and initial number of leapfrog steps. This (and other algorithmic details) imply that the step size must be a scalar.

In general, adaptation prevents the chain from reaching a stationary distribution, so obtaining consistent samples requires num_adaptation_steps be set to a value [somewhat smaller][1] than the number of burnin steps. However, it may sometimes be helpful to set num_adaptation_steps to a larger value during development in order to inspect the behavior of the chain during adaptation.

Examples

This implements something similar to ChEES HMC from [2].

import tensorflow as tf
import tensorflow_probability as tfp
tfb = tfp.bijectors
tfd = tfp.distributions

target_log_prob_fn = tfd.JointDistributionSequential([
    tfd.Normal(0., 20.),
    tfd.HalfNormal(10.),
]).log_prob

num_burnin_steps = 1000
num_adaptation_steps = int(num_burnin_steps * 0.8)
num_results = 500
num_chains = 16
step_size = 0.1

kernel = tfp.mcmc.HamiltonianMonteCarlo(
    target_log_prob_fn=target_log_prob_fn,
    step_size=step_size,
    num_leapfrog_steps=1,
)
kernel = tfp.experimental.mcmc.GradientBasedTrajectoryLengthAdaptation(
    kernel,
    num_adaptation_steps=num_adaptation_steps)
kernel = tfp.mcmc.DualAveragingStepSizeAdaptation(
    kernel, num_adaptation_steps=num_adaptation_steps)
kernel = tfp.mcmc.TransformedTransitionKernel(
    kernel,
    [tfb.Identity(),
     tfb.Exp()])

def trace_fn(_, pkr):
  return (
      pkr.inner_results.inner_results.inner_results.accepted_results
      .step_size,
      pkr.inner_results.inner_results.max_trajectory_length,
      pkr.inner_results.inner_results.inner_results.log_accept_ratio,
  )

# The chain will be stepped for num_results + num_burnin_steps, adapting for
# the first num_adaptation_steps.
samples, [step_size, max_trajectory_length, log_accept_ratio] = (
    tfp.mcmc.sample_chain(
        num_results=num_results,
        num_burnin_steps=num_burnin_steps,
        current_state=[tf.zeros(num_chains),
                       tf.zeros(num_chains)],
        kernel=kernel,
        trace_fn=trace_fn,))

# ~0.75
accept_prob = tf.math.exp(tfp.math.reduce_logmeanexp(
    tf.minimum(log_accept_ratio, 0.)))

References

[1]: <http://andrewgelman.com/2017/12/15/ burn-vs-warm-iterative-simulation-algorithms/#comment_627745>

[2]: Hoffman, M., Radul, A., & Sountsov, P. (2020). An Adaptive MCMC Scheme for Setting Trajectory Lengths in Hamiltonian Monte Carlo. In preparation.

Args
`inner_kernel`	`TransitionKernel`-like object.
`num_adaptation_steps`	Scalar `int` `Tensor` number of initial steps to during which to adjust the step size. This may be greater, less than, or equal to the number of burnin steps.
`use_halton_sequence_jitter`	Python bool. Whether to use a Halton sequence for jittering the trajectory length. This makes the procedure more stable than sampling trajectory lengths from a uniform distribution.
`adaptation_rate`	Floating point scalar `Tensor`. How rapidly to adapt the trajectory length.
`jitter_amount`	Floating point scalar `Tensor`. How much to jitter the trajectory on the next step. The trajectory length is sampled from `[(1 jitter_amount) * max_trajectory_length, max_trajectory_length]`. </td> </tr><tr> <td>`criterion_fn`</td> <td> Callable with`(previous_state, proposed_state, accept_prob) -> criterion`. Computes the criterion value. </td> </tr><tr> <td>`max_leapfrog_steps`</td> <td> Int32 scalar`Tensor`. Clips the number of leapfrog steps to this value. </td> </tr><tr> <td>`num_leapfrog_steps_getter_fn`</td> <td> A callable with the signature`(kernel_results) -> num_leapfrog_steps`where`kernel_results`are the results of the`inner_kernel`, and`num_leapfrog_steps`is a floating point`Tensor`. </td> </tr><tr> <td>`num_leapfrog_steps_setter_fn`</td> <td> A callable with the signature`(kernel_results, new_num_leapfrog_steps) -> new_kernel_results`where`kernel_results`are the results of the`inner_kernel`,`new_num_leapfrog_steps`is a scalar tensor`Tensor`, and`new_kernel_results`are a copy of`kernel_results`with the number of leapfrog steps set. </td> </tr><tr> <td>`step_size_getter_fn`</td> <td> A callable with the signature`(kernel_results) -> step_size`where`kernel_results`are the results of the`inner_kernel`, and`step_size`is a floating point`Tensor`. </td> </tr><tr> <td>`proposed_velocity_getter_fn`</td> <td> A callable with the signature`(kernel_results) -> proposed_velocity`where`kernel_results`are the results of the`inner_kernel`, and`proposed_velocity`is a (possibly nested) floating point`Tensor`. Velocity is derivative of state with respect to trajectory length. </td> </tr><tr> <td>`log_accept_prob_getter_fn`</td> <td> A callable with the signature`(kernel_results) -> log_accept_prob`where`kernel_results`are the results of the`inner_kernel`, and`log_accept_prob`is a floating point`Tensor`.`log_accept_prob`has shape`[C0, ...., Cb]`with`b > 0`. </td> </tr><tr> <td>`proposed_state_getter_fn`</td> <td> A callable with the signature`(kernel_results) -> proposed_state`where`kernel_results`are the results of the`inner_kernel`, and`proposed_state`is a (possibly nested) floating point`Tensor`. </td> </tr><tr> <td>`validate_args`</td> <td> Python`bool`. When`True`kernel parameters are checked for validity. When`False`invalid inputs may silently render incorrect outputs. </td> </tr><tr> <td>`experimental_shard_axis_names`</td> <td> A structure of string names indicating how members of the state are sharded. </td> </tr><tr> <td>`name`</td> <td> Python`str` name prefixed to Ops created by this class. Default: 'simple_step_size_adaptation'.

Raises
`ValueError`	If `inner_kernel` contains a `TransformedTransitionKernel` in its hierarchy. If you need to use the `TransformedTransitionKernel`, place it above this kernel in the hierarchy (see the example in the class docstring).

Attributes
`experimental_shard_axis_names`	The shard axis names for members of the state.
`inner_kernel`
`is_calibrated`	Returns `True` if Markov chain converges to specified distribution. `TransitionKernel`s which are "uncalibrated" are often calibrated by composing them with the `tfp.mcmc.MetropolisHastings` `TransitionKernel`.
`max_leapfrog_steps`
`name`
`num_adaptation_steps`
`parameters`	Return `dict` of `__init__` arguments and their values.
`use_halton_sequence_jitter`
`validate_args`

Methods

`bootstrap_results`

View source

bootstrap_results(
    init_state
)

Returns an object with the same type as returned by one_step(...)[1].

Args
`init_state`	`Tensor` or Python `list` of `Tensor`s representing the initial state(s) of the Markov chain(s).

Returns
`kernel_results`	A (possibly nested) `tuple`, `namedtuple` or `list` of `Tensor`s representing internal calculations made within this function.

`copy`

View source

copy(
    **override_parameter_kwargs
)

Non-destructively creates a deep copy of the kernel.

Args
`**override_parameter_kwargs`	Python String/value `dictionary` of initialization arguments to override with new values.

Returns
`new_kernel`	`TransitionKernel` object of same type as `self`, initialized with the union of self.parameters and override_parameter_kwargs, with any shared keys overridden by the value of override_parameter_kwargs, i.e., `dict(self.parameters, **override_parameters_kwargs)`.

`criterion_fn`

View source

criterion_fn(
    previous_state, proposed_state, accept_prob
)

`experimental_with_shard_axes`

View source

experimental_with_shard_axes(
    shard_axis_names
)

Returns a copy of the kernel with the provided shard axis names.

Args
`shard_axis_names`	a structure of strings indicating the shard axis names for each component of this kernel's state.

Returns
A copy of the current kernel with the shard axis information.

`log_accept_prob_getter_fn`

View source

log_accept_prob_getter_fn(
    kernel_results
)

`num_leapfrog_steps_getter_fn`

View source

num_leapfrog_steps_getter_fn(
    kernel_results
)

`num_leapfrog_steps_setter_fn`

View source

num_leapfrog_steps_setter_fn(
    kernel_results, new_num_leapfrog_steps
)

`one_step`

View source

one_step(
    current_state, previous_kernel_results, seed=None
)

Takes one step of the TransitionKernel.

Must be overridden by subclasses.

Args
`current_state`	`Tensor` or Python `list` of `Tensor`s representing the current state(s) of the Markov chain(s).
`previous_kernel_results`	A (possibly nested) `tuple`, `namedtuple` or `list` of `Tensor`s representing internal calculations made within the previous call to this function (or as returned by `bootstrap_results`).
`seed`	Optional, a seed for reproducible sampling.

Returns
`next_state`	`Tensor` or Python `list` of `Tensor`s representing the next state(s) of the Markov chain(s).
`kernel_results`	A (possibly nested) `tuple`, `namedtuple` or `list` of `Tensor`s representing internal calculations made within this function.

`proposed_state_getter_fn`

View source

proposed_state_getter_fn(
    kernel_results
)

`proposed_velocity_getter_fn`

View source

proposed_velocity_getter_fn(
    kernel_results
)

`step_size_getter_fn`

View source

step_size_getter_fn(
    kernel_results
)

tfp.experimental.mcmc.GradientBasedTrajectoryLengthAdaptation

Examples

References

Args

Raises

Attributes

Methods

bootstrap_results

copy

criterion_fn

experimental_with_shard_axes

log_accept_prob_getter_fn

num_leapfrog_steps_getter_fn

num_leapfrog_steps_setter_fn

one_step

proposed_state_getter_fn

proposed_velocity_getter_fn

step_size_getter_fn

`bootstrap_results`

`copy`

`criterion_fn`

`experimental_with_shard_axes`

`log_accept_prob_getter_fn`

`num_leapfrog_steps_getter_fn`

`num_leapfrog_steps_setter_fn`

`one_step`

`proposed_state_getter_fn`

`proposed_velocity_getter_fn`

`step_size_getter_fn`