Keras 3: Streamlined Backend

[LarsKue]

Work In Progress

This PR is still in progress. Please discuss open issues and raise possible concerns below.

Summary

• Streamline the definition of Prior and other distributions
• When sampling parameters, they are now always passed by name in a dictionary
• Add decorators for easy construction and batching of backend-conforming distribution objects

  
  @bf.distribution
  def prior():
  return {"theta": np.random.normal(size=2)}

batch_of_samples = prior.sample((32,)) assert batch_of_samples["theta"].shape == (32, 2)

- `Amortizer`s are now Keras 3 Models, which allows backend-agnostic training
```py3
# set backend to "tensorflow", "jax", or "torch" before importing bayesflow
import os
os.environ["KERAS_BACKEND"] = "torch"

import bayesflow as bf

amortizer = bf.AmortizedPosterior(...)
amortizer.fit()

• Move behavior of Configurator into Amortizer
• Add default configuration behavior for edge cases, like no summary network
• General overhaul of the data flow inside Amortizer
• Add a Dataset object that takes care of data loading in multiple worker processes
• Move Training Strategy from Trainer into Dataset

  # online
  dataset = bf.datasets.OnlineDataset(generative_model=..., workers=12, use_multiprocessing=True)
  amortizer.fit(dataset, steps_per_epoch=1000)

  # offline, in memory
  data = {...}  # some dictionary
  dataset = OfflineDataset(data, workers=12, use_multiprocessing=True)
  amortizer.fit(dataset)

  
  # offline, on disk
  import keras

class MyDataset(keras.utils.PyDataset): ... # user-implemented data loading from disk

dataset = MyDataset(workers=12, use_multiprocessing=True) amortizer.fit(dataset)

# In Progress

- Allow the user to specify what variables are observed vs. inferred vs. conditioned on by name
```py3
# two moons
posterior = Amortizer(
    inference_network,
    observed_variables=["x1", "x2"],
    inferred_variables=["theta1", "theta2"],
    inference_conditions=["r", "alpha"]
)

• Named arguments with distribution decorators (@LarsKue)

  @bf.Prior(is_conditional=True)
  def prior(alpha, beta):
  return {"a": 2 * alpha, "b": alpha + beta}

• Update Documentation (@LarsKue, mostly done)
• Implement hierarchical amortizers for multi-level models (@daniel-habermann)
• Port existing networks to Keras 3 (@chase-grajeda)
• Add support for non-batchable context and make this the default (@LarsKue)

Postponed

We should probably do these things after merging with dev and before merging with main:

• Add support for graph structured priors
• Split sampled data by parameter names and return as a dict (@jerrymhuang)
• Constrain predicted parameters to user-defined subspaces (@Kucharssim)
• Add coverage statistics to README.md with a workflow
• Expand test coverage to at least 75%
• Update example notebooks
• • Add smart defaults (@stefanradev93)

    settings = bf.settings.propose(
    training="offline",
    dataset_size=600_000,
    data_shape=(200, 2),
    data_type="time series",
    parameters_shape=(8,)
    )

In Discussion

• Add a WorkFlow (name wip) object that encapsulates both amortizer and dataset for easier post-processing and model sharing
• Rename batchable / non-batchable context

Dropped

• Allow the user to pass a dictionary of distributions instead of defining a prior

  import tensorflow_probability as tfp
  D = tfp.distributions
  prior = {"theta": D.Normal([0.0, 0.0], [1.0, 1.0])}

  Reason: We should enforce a single way to do things. Also poor support with pure jax.

• Return sampled data as a DataFrame

Reason: Too restrictive for data structure.

[paul-buerkner]

@LarsKue Thank you so much! This already looks amazing!

Could you perhaps add a simple fully runnable example here for people to get started playing around with it? It is kind of there above, but I think it would make things easier to have one chunk of example code to copy and edit from there.

Everyone, please try out the new interface and tell us what you think!

[LarsKue]

@paul-buerkner Yes, I am working on it. I hope I have one ready today.

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[marvinschmitt]

Great work!! 👏

I'll write down some thoughts on the installation process. Those don't need any changes in the streamlined codebase but are just reminders for our future selves shortly before the release.

• In addition to keras (which will replace the current tensorflow dependencies), the user has to install their favorite backend. How do we approach this for users who aren't proficient in Python-ecosystem stuff? A few initial thoughts:
  • Extras like pip install bayesflow[torch]. Advantage: Easy interface. Disadvantage: Restricted to pip, no mamba equivalent. I don't like that option.
  • Message during the bayesflow installation. That's annoying (if possible at all?) and I wouldn't do that either.
  • After installation, upon running bayesflow: Catch any errors that relate to missing backend packages and provide a comprehensive error message with concrete pointers on how to install the necessary backend to fix the issue. Currently my favorite option.
• Python >=3.11 is required for typing.Self

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