Note
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Example: ProdLDA with Flax
In this example, we will follow [1] to implement the ProdLDA topic model from Autoencoding Variational Inference For Topic Models by Akash Srivastava and Charles Sutton [2]. This model returns consistently better topics than vanilla LDA and trains much more quickly. Furthermore, it does not require a custom inference algorithm that relies on complex mathematical derivations. This example also serves as an introduction to Flax modules in NumPyro.
Note that unlike [1, 2], this implementation uses a Dirichlet prior directly rather than approximating it with a softmax-normal distribution.
For the interested reader, a nice extension of this model is the CombinedTM model [3] which utilizes a pre-trained sentence transformer (like https://www.sbert.net/) to generate a better representation of the encoded latent vector.
- References:
Akash Srivastava, & Charles Sutton. (2017). Autoencoding Variational Inference For Topic Models.
Federico Bianchi, Silvia Terragni, and Dirk Hovy (2021), “Pre-training is a Hot Topic: Contextualized Document Embeddings Improve Topic Coherence” (https://arxiv.org/abs/2004.03974)
import argparse
import matplotlib.pyplot as plt
import pandas as pd
from sklearn.datasets import fetch_20newsgroups
from sklearn.feature_extraction.text import CountVectorizer
from wordcloud import WordCloud
import flax.linen as nn
import jax
from jax import device_put, random
import jax.numpy as jnp
import numpyro
from numpyro.contrib.module import flax_module
import numpyro.distributions as dist
from numpyro.infer import SVI, TraceMeanField_ELBO
class FlaxEncoder(nn.Module):
vocab_size: int
num_topics: int
hidden: int
dropout_rate: float
@nn.compact
def __call__(self, inputs, is_training):
h = nn.softplus(nn.Dense(self.hidden)(inputs))
h = nn.softplus(nn.Dense(self.hidden)(h))
h = nn.Dropout(self.dropout_rate, deterministic=not is_training)(h)
h = nn.Dense(self.num_topics)(h)
log_concentration = nn.BatchNorm(
use_bias=False,
use_scale=False,
momentum=0.9,
use_running_average=not is_training,
)(h)
return jnp.exp(log_concentration)
class FlaxDecoder(nn.Module):
vocab_size: int
dropout_rate: float
@nn.compact
def __call__(self, inputs, is_training):
h = nn.Dropout(self.dropout_rate, deterministic=not is_training)(inputs)
h = nn.Dense(self.vocab_size, use_bias=False)(h)
return nn.BatchNorm(
use_bias=False,
use_scale=False,
momentum=0.9,
use_running_average=not is_training,
)(h)
def model(docs, hyperparams, is_training=False):
decoder = flax_module(
"decoder",
FlaxDecoder(hyperparams["vocab_size"], hyperparams["dropout_rate"]),
input_shape=(1, hyperparams["num_topics"]),
# ensure PRNG key is made available to dropout layers
apply_rng=["dropout"],
# indicate mutable state due to BatchNorm layers
mutable=["batch_stats"],
# to ensure proper initialisation of BatchNorm we must
# initialise with is_training=True
is_training=True,
)
with numpyro.plate(
"documents", docs.shape[0], subsample_size=hyperparams["batch_size"]
):
batch_docs = numpyro.subsample(docs, event_dim=1)
theta = numpyro.sample(
"theta", dist.Dirichlet(jnp.ones(hyperparams["num_topics"]))
)
logits = decoder(theta, is_training, rngs={"dropout": numpyro.prng_key()})
total_count = batch_docs.sum(-1)
numpyro.sample(
"obs", dist.Multinomial(total_count, logits=logits), obs=batch_docs
)
def guide(docs, hyperparams, is_training=False):
encoder = flax_module(
"encoder",
FlaxEncoder(
hyperparams["vocab_size"],
hyperparams["num_topics"],
hyperparams["hidden"],
hyperparams["dropout_rate"],
),
input_shape=(1, hyperparams["vocab_size"]),
# ensure PRNG key is made available to dropout layers
apply_rng=["dropout"],
# indicate mutable state due to BatchNorm layers
mutable=["batch_stats"],
# to ensure proper initialisation of BatchNorm we must
# initialise with is_training=True
is_training=True,
)
with numpyro.plate(
"documents", docs.shape[0], subsample_size=hyperparams["batch_size"]
):
batch_docs = numpyro.subsample(docs, event_dim=1)
concentration = encoder(
batch_docs, is_training, rngs={"dropout": numpyro.prng_key()}
)
numpyro.sample("theta", dist.Dirichlet(concentration))
def load_data():
news = fetch_20newsgroups(subset="all")
vectorizer = CountVectorizer(max_df=0.5, min_df=20, stop_words="english")
docs = jnp.array(vectorizer.fit_transform(news["data"]).toarray())
vocab = pd.DataFrame(columns=["word", "index"])
vocab["word"] = vectorizer.get_feature_names_out()
vocab["index"] = vocab.index
return docs, vocab
def run_inference(docs, args):
rng_key = random.key(0)
docs = device_put(docs)
hyperparams = dict(
vocab_size=docs.shape[1],
num_topics=args.num_topics,
hidden=args.hidden,
dropout_rate=args.dropout_rate,
batch_size=args.batch_size,
)
optimizer = numpyro.optim.Adam(args.learning_rate)
svi = SVI(model, guide, optimizer, loss=TraceMeanField_ELBO())
return svi.run(
rng_key,
args.num_steps,
docs,
hyperparams,
is_training=True,
progress_bar=not args.disable_progbar,
)
def plot_word_cloud(b, ax, vocab, n):
indices = jnp.argsort(b)[::-1]
top20 = indices[:20]
df = pd.DataFrame(top20, columns=["index"])
words = pd.merge(df, vocab[["index", "word"]], how="left", on="index")[
"word"
].values.tolist()
sizes = b[top20].tolist()
freqs = {words[i]: sizes[i] for i in range(len(words))}
wc = WordCloud(background_color="white", width=800, height=500)
wc = wc.generate_from_frequencies(freqs)
ax.set_title(f"Topic {n + 1}")
ax.imshow(wc, interpolation="bilinear")
ax.axis("off")
def main(args):
docs, vocab = load_data()
print(f"Dictionary size: {len(vocab)}")
print(f"Corpus size: {docs.shape}")
svi_result = run_inference(docs, args)
beta = svi_result.params["decoder$params"]["Dense_0"]["kernel"]
beta = jax.nn.softmax(beta)
# the number of plots depends on the chosen number of topics.
# add 2 to num topics to ensure we create a row for any remainder after division
nrows = (args.num_topics + 2) // 3
fig, axs = plt.subplots(nrows, 3, figsize=(14, 3 + 3 * nrows))
axs = axs.flatten()
for n in range(beta.shape[0]):
plot_word_cloud(beta[n], axs[n], vocab, n)
# hide any unused axes
for i in range(n, len(axs)):
axs[i].axis("off")
fig.savefig("wordclouds.png")
if __name__ == "__main__":
assert numpyro.__version__.startswith("0.20.1")
parser = argparse.ArgumentParser(
description="Probabilistic topic modelling with Flax"
)
parser.add_argument("-n", "--num-steps", nargs="?", default=30_000, type=int)
parser.add_argument("-t", "--num-topics", nargs="?", default=12, type=int)
parser.add_argument("--batch-size", nargs="?", default=32, type=int)
parser.add_argument("--learning-rate", nargs="?", default=1e-3, type=float)
parser.add_argument("--hidden", nargs="?", default=200, type=int)
parser.add_argument("--dropout-rate", nargs="?", default=0.2, type=float)
parser.add_argument(
"-dp",
"--disable-progbar",
action="store_true",
default=False,
help="Whether to disable progress bar",
)
parser.add_argument(
"--device", default="cpu", type=str, help='use "cpu", "gpu" or "tpu".'
)
args = parser.parse_args()
numpyro.set_platform(args.device)
main(args)
