# Normalizing Flows Overview¶

Normalizing Flows is a rich family of distributions. They were described by Rezende and Mohamed, and their experiments proved the importance of studying them further. Some extensions like that of Tomczak and Welling made partially/full rank Gaussian approximations for high dimensional spaces computationally tractable.

This notebook reveals some tips and tricks for using normalizing flows effectively in PyMC3.

In :

%matplotlib inline
import matplotlib.pyplot as plt
import seaborn as sns
import pymc3 as pm
import numpy as np
from theano import theano, tensor as tt
from collections import Counter

pm.set_tt_rng(42)
np.random.seed(42)


## Theory¶

Normalizing flows is a series of invertible transformations on an initial distribution.

$z_K = f_K \circ \dots \circ f_2 \circ f_1(z_0)$

In this case, we can compute a tractable density for the flow.

$\ln q_K(z_K) = \ln q_0(z_0) - \sum_{k=1}^{K}\ln \left|\frac{\partial f_k}{\partial z_{k-1}}\right|$

Here, every $$f_k$$ is a parametric function with a well-defined determinant. The transformation used is up to the user; for example, the simplest flow is an affine transform:

$z = loc(scale(z_0)) = \mu + \sigma * z_0$

In this case, we get a mean field approximation if $$z_0 \sim \mathcal{N}(0, 1)$$

## Flow Formulas¶

In PyMC3 there are flexible ways to define flows with formulas. There are currently 5 types defined:

• Loc (loc): $$z' = z + \mu$$
• Scale (scale): $$z' = \sigma * z$$
• Planar (planar): $$z' = z + u * \tanh(w^T z + b)$$
• Radial (radial): $$z' = z + \beta (\alpha + ||z-z_r||)^{-1}(z-z_r)$$
• Householder (hh): $$z' = H z$$

Formulae can be composed as a string, e.g. 'scale-loc', 'scale-hh*4-loc', 'panar*10'. Each step is separated with '-', and repeated flows are defined with '*' in the form of '<flow>*<#repeats>'.

Flow-based approximations in PyMC3 are based on the NormalizingFlow class, with corresponding inference classes named using the NF abbreviation (analogous to how ADVI and SVGD are treated in PyMC3).

Concretely, an approximation is represented by:

In :

pm.NormalizingFlow

Out:

pymc3.variational.approximations.NormalizingFlow


While an inference class is:

In :

pm.NFVI

Out:

pymc3.variational.inference.NFVI


## Flow patterns¶

Composing flows requires some understanding of the target output. Flows that are too complex might not converge, whereas if they are too simple, they may not accurately estimate the posterior.

Let’s start simply:

In :

with pm.Model() as dummy:

N = pm.Normal('N', shape=(100,))


### Mean Field connectivity¶

Let’s apply the transformation corresponding to the mean-field family to begin with:

In :

pm.NormalizingFlow('scale-loc', model=dummy)

Out:

<pymc3.variational.approximations.NormalizingFlow at 0x109b841d0>


### Full Rank Normal connectivity¶

We can get a full rank model with dense covariance matrix using householder flows (hh). One hh flow adds exactly one rank to the covariance matrix, so for a full rank matrix we need K=ndim householder flows. hh flows are volume-preserving, so we need to change the scaling if we want our posterior to have unit variance for the latent variables.

After we specify the covariance with a combination of 'scale-hh*K', we then add location shift with the loc flow. We now have a full-rank analog:

In :

pm.NormalizingFlow('scale-hh*100-loc', model=dummy)

Out:

<pymc3.variational.approximations.NormalizingFlow at 0x1156df0b8>


A more interesting case is when we do not expect a lot of interactions within the posterior. In this case, where our covariance is expected to be sparse, we can constrain it by defining a low rank approximation family.

This has the additional benefit of reducing the computational cost of approximating the model.

In :

pm.NormalizingFlow('scale-hh*10-loc', model=dummy)

Out:

<pymc3.variational.approximations.NormalizingFlow at 0x1156dfeb8>


Parameters can be initialized randomly, using the jitter argument to specify the scale of the randomness.

In :

pm.NormalizingFlow('scale-hh*10-loc', model=dummy, jitter=.001) # LowRank

Out:

<pymc3.variational.approximations.NormalizingFlow at 0x115fea7f0>


• Planar (planar): $$z' = z + u * \tanh(w^T z + b)$$
• Radial (radial): $$z' = z + \beta (\alpha + ||z-z_r||)^{-1}(z-z_r)$$

Planar flows are useful for splitting the incoming distribution into two parts, which allows multimodal distributions to be modeled.

Similarly, a radial flow changes density around a specific reference point.

## Simulated data example¶

There were 4 potential functions illustrated in the original paper, which we can replicate here. Inference can be unstable in multimodal cases, but there are strategies for dealing with them.

First, let’s specify the potential functions:

In :

def w1(z):
return tt.sin(2.*np.pi*z/4.)
def w2(z):
return 3.*tt.exp(-.5*(((z-1.)/.6))**2)
def w3(z):
return 3.*(1+tt.exp(-(z-1.)/.3))**-1

def pot1(z):
z = z.T
return .5*((z.norm(2, axis=0)-2.)/.4)**2 - tt.log(tt.exp(-.5*((z-2.)/.6)**2) +
tt.exp(-.5*((z+2.)/.6)**2))
def pot2(z):
z = z.T
return .5*((z-w1(z))/.4)**2 + 0.1*tt.abs_(z)

def pot3(z):
z = z.T
return -tt.log(tt.exp(-.5*((z-w1(z))/.35)**2) +
tt.exp(-.5*((z-w1(z)+w2(z))/.35)**2)) + 0.1*tt.abs_(z)

def pot4(z):
z = z.T
return -tt.log(tt.exp(-.5*((z-w1(z))/.4)**2) +
tt.exp(-.5*((z-w1(z)+w3(z))/.35)**2)) + 0.1*tt.abs_(z)

z = tt.matrix('z')
z.tag.test_value = pm.floatX([[0., 0.]])
pot1f = theano.function([z], pot1(z))
pot2f = theano.function([z], pot2(z))
pot3f = theano.function([z], pot3(z))
pot4f = theano.function([z], pot4(z))

In :

def contour_pot(potf, ax=None, title=None, xlim=5, ylim=5):
grid = pm.floatX(np.mgrid[-xlim:xlim:100j,-ylim:ylim:100j])
grid_2d = grid.reshape(2, -1).T
cmap = plt.get_cmap('inferno')
if ax is None:
_, ax = plt.subplots(figsize=(12, 9))
pdf1e = np.exp(-potf(grid_2d))
contour = ax.contourf(grid, grid, pdf1e.reshape(100, 100), cmap=cmap)
if title is not None:
ax.set_title(title, fontsize=16)
return ax

In :

fig, ax = plt.subplots(2, 2, figsize=(12, 12))
ax = ax.flatten()
contour_pot(pot1f, ax, 'pot1', );
contour_pot(pot2f, ax, 'pot2');
contour_pot(pot3f, ax, 'pot3');
contour_pot(pot4f, ax, 'pot4');
fig.tight_layout() ## Reproducing first potential function¶

In :

from pymc3.distributions.dist_math import bound
def cust_logp(z):
#return bound(-pot1(z), z>-5, z<5)
return -pot1(z)

with pm.Model() as pot1m:
pm.DensityDist('pot1', logp=cust_logp, shape=(2,))


### NUTS¶

Let’s use NUTS first. Just to have a look how good is it’s approximation.

Note you may need to rerun the model a couple of times, as the sampler/estimator might not fully explore function due to multimodality.
In :

pm.set_tt_rng(42)
np.random.seed(42)
with pot1m:
trace = pm.sample(1000, init='auto', njobs=2, start=[dict(pot1=np.array([-2, 0])),
dict(pot1=np.array([2, 0]))])

Auto-assigning NUTS sampler...
99%|█████████▊| 1481/1500 [00:03<00:00, 473.27it/s]/Users/ferres/dev/pymc3/pymc3/step_methods/hmc/nuts.py:451: UserWarning: The acceptance probability in chain 0 does not match the target. It is 0.889185534675, but should be close to 0.8. Try to increase the number of tuning steps.
% (self._chain_id, mean_accept, target_accept))
100%|██████████| 1500/1500 [00:03<00:00, 467.67it/s]

In :

dftrace = pm.trace_to_dataframe(trace)
sns.jointplot(dftrace.iloc[:, 0], dftrace.iloc[:, 1], kind='kde')

Out:

<seaborn.axisgrid.JointGrid at 0x116b9a208> ### Normalizing flows¶

As a first (naive) try with flows, we will keep things simple: Let’s use just 2 planar flows and see what we get:

In :

with pot1m:
inference = pm.NFVI('planar*2', jitter=1)

## Plotting starting distribution
dftrace = pm.trace_to_dataframe(inference.approx.sample(1000))
sns.jointplot(dftrace.iloc[:, 0], dftrace.iloc[:, 1], kind='kde'); It is illustrative to track gradients as well as parameters. In this setup, different sampling points can give different gradients because a single sampled point tends to collapse to a mode.

Here are the parameters of the model:

In :

inference.approx.params

Out:

[b, u, w, b, u, w]


We also require an objective:

In :

inference.objective(nmc=None)

Out:

Elemwise{mul,no_inplace}.0


Theano can be used to calcuate the gradient of the objective with respect to the parameters:

In :

with theano.configparser.change_flags(compute_test_value='off'):

Out:

[Elemwise{add,no_inplace}.0,


If we want to keep track of the gradient changes during the inference, we warp them in a pymc3 callback:

In :

from collections import defaultdict, OrderedDict
from itertools import count

@theano.configparser.change_flags(compute_test_value='off')
def get_tracker(inference):
numbers = defaultdict(count)
params = inference.approx.params
names = ['%s_%d' % (v.name, next(numbers[v.name])) for v in inference.approx.params]
return pm.callbacks.Tracker(**OrderedDict(
[(name, v.eval) for name, v in zip(names, params)] + [('grad_' + name, v.eval) for name, v in zip(names, grads)]
))

tracker = get_tracker(inference)

In :

tracker.whatchdict

Out:

{'b_0': <bound method Variable.eval of b>,
'b_1': <bound method Variable.eval of b>,
'u_0': <bound method Variable.eval of u>,
'u_1': <bound method Variable.eval of u>,
'w_0': <bound method Variable.eval of w>,
'w_1': <bound method Variable.eval of w>}

In :

inference.fit(30000, obj_optimizer=pm.adagrad_window(learning_rate=.01), callbacks=[tracker])

Average Loss = -0.7863: 100%|██████████| 30000/30000 [00:55<00:00, 541.29it/s]
Finished [100%]: Average Loss = -0.79931

Out:

<pymc3.variational.approximations.NormalizingFlow at 0x1166a64e0>

In :

dftrace = pm.trace_to_dataframe(inference.approx.sample(1000))
sns.jointplot(dftrace.iloc[:, 0], dftrace.iloc[:, 1], kind='kde')

Out:

<seaborn.axisgrid.JointGrid at 0x11e519860> In :

plt.plot(inference.hist); As you can see, the objective history is not very informative here. This is where the gradient tracker can be more informative.

In :

trackername = ['u_0', 'w_0', 'b_0', 'u_1', 'w_1', 'b_1',

def plot_tracker_results(tracker):
fig, ax = plt.subplots(len(tracker.hist)//2, 2, figsize=(16, len(tracker.hist)//2*2.3))
ax = ax.flatten()
#names = list(tracker.hist.keys())
names = trackername
gnames = names[len(names)//2:]
names = names[:len(names)//2]
pairnames = zip(names, gnames)
i = names.index(name)
left = ax[i*2]
right = ax[i*2+1]
params = np.asarray(tracker[name])
if params.ndim == 1:
params = params[:, None]
params = params.T
left.set_title('Param trace of %s' % name)
s = params.shape
for j, (v, g) in enumerate(zip(params, grads)):
left.plot(v, '-')
right.plot(g, 'o', alpha=1/s/10)
left.legend([name + '_%d' % j for j in range(len(names))])
right.legend([gname + '_%d' % j for j in range(len(names))])

for vn, gn in pairnames:
fig.tight_layout()

In :

plot_tracker_results(tracker); Inference is often unstable, some parameters are not well fitted as they poorly influence the resulting posterior.

In a multimodal setting, the dominant mode might well change from run to run.

### Going deeper¶

We can try to improve our approximation by adding flows; in the original paper they used both 8 and 32. Let’s try using 8 here.

In :

with pot1m:
inference = pm.NFVI('planar*8', jitter=1.)

dftrace = pm.trace_to_dataframe(inference.approx.sample(1000))
sns.jointplot(dftrace.iloc[:, 0], dftrace.iloc[:, 1], kind='kde'); We can try for a more robust fit by allocating more samples to obj_n_mc in fit, which controls the number of Monte Carlo samples used to approximate the gradient.

In :

inference.fit(25000, obj_optimizer=pm.adam(learning_rate=.01), obj_n_mc=100,
callbacks=[pm.callbacks.CheckParametersConvergence()])

Average Loss = -1.7429: 100%|██████████| 25000/25000 [01:56<00:00, 215.40it/s]
Finished [100%]: Average Loss = -1.7428

Out:

<pymc3.variational.approximations.NormalizingFlow at 0x11cfa3198>

In :

dftrace = pm.trace_to_dataframe(inference.approx.sample(1000))
sns.jointplot(dftrace.iloc[:, 0], dftrace.iloc[:, 1], kind='kde')

Out:

<seaborn.axisgrid.JointGrid at 0x122108ac8> This is a noticeable improvement. Here, we see that flows are able to characterize the multimodality of a given posterior, but as we have seen, they are hard to fit. The initial point of the optimization matters in general for the multimodal case.

### MCMC vs NFVI¶

Let’s use another potential function, and compare the sampling using NUTS to what we get with NF:

In :

def cust_logp(z):
return -pot4(z)

with pm.Model() as pot_m:
pm.DensityDist('pot_func', logp=cust_logp, shape=(2,))

In :

with pot_m:
traceNUTS = pm.sample(3000, tune=1000, target_accept=0.9, njobs=2)

Auto-assigning NUTS sampler...
99%|█████████▉| 3969/4000 [00:09<00:00, 393.42it/s]/Users/ferres/dev/pymc3/pymc3/step_methods/hmc/nuts.py:467: UserWarning: Chain 0 contains 94 diverging samples after tuning. If increasing target_accept does not help try to reparameterize.
% (self._chain_id, n_diverging))
100%|██████████| 4000/4000 [00:09<00:00, 402.32it/s]
/Users/ferres/dev/pymc3/pymc3/step_methods/hmc/nuts.py:451: UserWarning: The acceptance probability in chain 1 does not match the target. It is 0.632933350266, but should be close to 0.8. Try to increase the number of tuning steps.
% (self._chain_id, mean_accept, target_accept))
/Users/ferres/dev/pymc3/pymc3/step_methods/hmc/nuts.py:467: UserWarning: Chain 1 contains 84 diverging samples after tuning. If increasing target_accept does not help try to reparameterize.
% (self._chain_id, n_diverging))

In :

formula = 'planar*10'
with pot_m:
inference = pm.NFVI(formula, jitter=0.1)

traceNF = inference.approx.sample(5000)

Average Loss = -2.212: 100%|██████████| 25000/25000 [01:05<00:00, 381.94it/s]
Finished [100%]: Average Loss = -2.2131

In :

fig, ax = plt.subplots(1, 3, figsize=(18, 6))
contour_pot(pot4f, ax, 'Target Potential Function');

ax.scatter(traceNUTS['pot_func'][:,0], traceNUTS['pot_func'][:,1],c='r',alpha=.05)
ax.set_xlim(-5,5)
ax.set_ylim(-5,5)
ax.set_title('NUTS')

ax.scatter(traceNF['pot_func'][:,0], traceNF['pot_func'][:,1],c='b',alpha=.05)
ax.set_xlim(-5,5)
ax.set_ylim(-5,5)
ax.set_title('NF with ' + formula); 