JAX-compatible interfaces with ML-surrogates of physics models

This section discusses a variety of options for building JAX-friendly interfaces to surrogate models.

As an illustrative example, suppose we have a new neural network surrogate transport model that we would like to use in TORAX. Assume that all the boilerplate described in the previous sections has been taken care of, as well as the definition of some functions to convert between TORAX structures and tensors for the neural network.

class MyCustomSurrogateTransportModel(TransportModel):
  ...
  def _call_implementation(
      self,
      runtime_params: runtime_params_slice.RuntimeParams,
      geo: geometry.Geometry,
      core_profiles: state.CoreProfiles,
  ) -> TurbulentTransport:
    input_tensor = self._prepare_input(runtime_params, geo, core_profiles)

    output_tensor = self._call_surrogate_model(input_tensor)

    chi_i, chi_e, d_e, v_e = self._parse_output(output_tensor)

    return TurbulentTransport(
        chi_face_ion=chi_i,
        chi_face_electron=chi_e,
        d_e=d_e,
        v_e=v_e,
    )

In this guide, we explore a few options for how you could make the _call_surrogate_model function for an existing surrogate, while maintaining the full power of JAX:

  1. Manually reimplementing the model in JAX.

  2. Converting a Pytorch model to a JAX model.

  3. Using an ONNX model.

Note

These conversion methods are necessary in order to make an external model compatible with JAX’s autodiff and JIT functionality, which is required for using TORAX’s gradient-driven nonlinear solvers (e.g. Newton-Raphson). Interfacing with non-differentiable, non-JITtable models is possible (for an example, see the |QuaLiKiz| transport model implementation) if the linear solver is used. However, note that if the model is called within the step function, JIT will need to be disabled with JAX_DISABLE_JIT=True.

Option 1: manually reimplementing the model in JAX

If the architecture of the surrogate is sufficiently simple, you might consider reimplementing the model in JAX. The surrogates in TORAX are mostly implemented using Flax Linen, and can be found in the google-deepmind/fusion_surrogates repository. If you’re not familiar with Flax, you can check out the Flax documentation on how to define your own models.

Consider a PyTorch neural network,

import torch

class PyTorchMLP(torch.nn.Module):
  def __init__(self, hidden_dim: int, n_hidden: int, output_dim: int, input_dim: int):
    super().__init__()
    self.model = torch.nn.Sequential(
      torch.nn.Linear(input_dim, hidden_dim),
      torch.nn.ReLU(),
      *[torch.nn.Sequential(
        torch.nn.Linear(hidden_dim, hidden_dim),
        torch.nn.ReLU()
      ) for _ in range(n_hidden)],
      torch.nn.Linear(hidden_dim, output_dim)
    )

  def forward(self, x):
    return self.model(x)

torch_model = PyTorchMLP(hidden_dim, n_hidden, output_dim, input_dim)

This model can be replicated in Flax as follows:

from flax import linen

class FlaxMLP(linen.Module):
  hidden_dim: int
  n_hidden: int
  output_dim: int
  input_dim: int

@linen.compact
def __call__(self, x):
  x = linen.Dense(self.hidden_dim)(x)
  x = linen.relu(x)
  for _ in range(self.n_hidden):
    x = linen.Dense(self.hidden_dim)(x)
    x = linen.relu(x)
  x = linen.Dense(self.output_dim)(x)
  return x

flax_model = FlaxMLP(hidden_dim, n_hidden, output_dim, input_dim)

As this is only the model architecture, we need to load the trained weights separately. This can be a bit fiddly as you have to map from the parameter names in the weights checkpoint file to the parameter names in the Flax model.

For loading weights from a PyTorch checkpoint, you might do something like:

import torch

state_dict = torch.load(PYTORCH_CHECKPOINT_PATH)

params = {}
for i in range(n_hidden_layers):
  layer_dict = {
    "kernel": jnp.array(
      state_dict[f"model.{i*2}.weight"]
    ).T,
    "bias": jnp.array(
      pytorch_state_dict[f"model.{i*2}.bias"]
    ).T,
  }
  params[f"Dense_{i}"] = layer_dict

params = {'params': params}

The model can then be called like any Flax model,

output_tensor = jax.jit(flax_model.apply)(params, input_tensor)

Warning

You need to be very careful when loading from a PyTorch state dict, as Flax and PyTorch may have slightly different representations of the weights (for example, one could be the transpose of the other). It’s worth validating the output of your PyTorch model against your JAX model to make sure.

Option 2: converting a PyTorch model to a JAX model

Warning

The torch_xla2 package is still evolving, which means there may be unexpected breaking changes. Some of the methods described in this section may become deprecated with little warning.

If your model is in PyTorch, you could also consider using the torch_xla2 package to do the conversion to JAX automatically.

import torch
import torch_xla2 as tx

trained_model = torch.load(PYTORCH_MODEL_PATH, weights_only=False)  # Use weights_only=False if you want to load the full model
params, jax_model_from_torch = tx.extract_jax(model)

The model can then be called as a pure JAX function:

output_tensor = jax.jit(jax_model_from_torch)(params, input_tensor)

To remove the need for performing the conversion every time the model is loaded, you might want to save a JAX-compatible version of the weights and model to disk:

import jax
import numpy as np

# jax.export uses StableHLO to serialize the model to a binary format
exported_model = jax.export(jax.jit(jax_model_from_torch))
with open("model.hlo", "wb") as f:
  f.write(exported_model.serialize())

# The weights can be saved as numpy arrays
np.savez("weights.npz", *params)

The model can then be loaded and run as follows:

# Load the HLO checkpoint
with open('model.hlo', 'rb') as f:
  model_as_bytes = f.read()
  model = jax.export.deserialize(model_as_bytes)

# Load the weights
weights_as_npz = np.load('weights.npz')
weights = [jnp.array(v) for v in weights_as_npz.values()]

Option 3: using an ONNX model

The Open Neural Network Exchange format (ONNX) is a highly interoperable format for sharing neural network models. ONNX files include the model architecture and weights bundled together.

An ONNX model can be loaded and called as follows, making sure to specify the correct input and output node names for your specific model:

import onnxruntime as ort
import numpy as np

s = ort.InferenceSession(ONNX_MODEL_PATH)
onnx_output_tensor = s.run(
  # Output node names
  ['output1', 'output2'],
  # Mapping from input node names to input tensors
  # NOTE: input tensors must have correct dtype for your specific model
  {'input': np.asarray(input_tensor, dtype=np.float32)},
)

However, JAX will not be able to differentiate through the InferenceSession. To convert the ONNX model to a JAX representation, you can use the jaxonnxruntime package:

import jax.numpy as jnp
from jaxonnxruntime.backend import Backend as ONNXJaxBackend
import onnx

onnx_model = onnx.load_model(ONNX_MODEL_PATH)

jax_model_from_onnx = ONNXJaxBackend.prepare(onnx_model)
# NOTE: run() returns a list of output tensors, in order of the output nodes
output_tensors = jax.jit(jax_model_from_onnx.run)({"input": jnp.asarray(input_tensor, dtype=jnp.float32)})

Option 4: using a JAX callback

For more information see Using Jax in TORAX.

Best practices

Caching and lazy loading: Ideally, the model should be constructed and weights loaded once only, on the first call to the function. The loaded model should be cached and reused for subsequent calls.

For example, in the _combined function of the QLKNN transport model (the function that actually evaluates this model), we have:

model = get_model(self._model_path)
...
model_output = model.predict(...)

where

@functools.lru_cache(maxsize=1)
def get_model(path: str) -> base_qlknn_model.BaseQLKNNModel:
  """Load the model."""
  ...
  return qlknn_10d.QLKNN10D(path)

By decorating with functools.lru_cache(maxsize=1), the result of this function - the loaded model - is stored in the cache and is only re-loaded if the function is called with a different path.

JITting model calls: In general, you should make sure that your forward call of the model is JITted:

output_tensor = jax.jit(flax_model.apply)(params, input_tensor)  # Good
output_tensor = flax_model.apply(params, input_tensor)  # Bad

This is vital to ensure fast performance.