[CharlelieLrt]

Description

This PR adds CheckpointStateBundle so users can restart distillation with
their named models, optimizers, loss state, and optional random state.
The bundle composes the existing PhysicsNeMo checkpoint utilities without
changing their public API or saving private FastGen adapters:

bundle = CheckpointStateBundle(
    {"student": student, "fake_score": fake_score},
    optimizers={"student": student_optimizer, "fake_score": fake_optimizer},
    stateful={"loss": loss_fn},
)
bundle.save_checkpoint("checkpoints", epoch=20, include_rng=True)
bundle.load_checkpoint("checkpoints", epoch=20, restore_rng=True)

Users recreate matching components before loading and save at completed
optimizer updates. Distributed callers invoke the bundle on every rank and
restart with the same topology. The optional strategy in the next PR uses this
same bundle.

Checklist

• This PR follows the contribution guidelines.
• The documentation describes the API and its role in the stack.
• This PR remains a draft.

Dependencies

• Depends on

  • PhysicsNeMo #2202 extends the loss API to DMD2 and f-distill. This is the immediate prerequisite.

• Earlier prerequisites

  • PhysicsNeMo #2199 adds named diffusion inputs and outputs.
  • PhysicsNeMo #2200 adds inverse SNR and TrigFlow components.
  • PhysicsNeMo #2201 adds private adapters and public sCM and MeanFlow losses.

• Depends on this PR

  • PhysicsNeMo #2204 adds the optional training strategy and the grouped changelog entry. It follows this PR directly in the stack.

• Parallel FastGen work

  • FastGen #43 allows the NumPy 2 dependency used by PhysicsNeMo. PhysicsNeMo Add sCM and MeanFlow distillation losses #2201 uses an immutable fork pin, so it does not need to wait for this proposal to merge.
  • FastGen #44 provides shared loss hooks for external training loops. The initial PhysicsNeMo integration does not require these hooks.

• Conditional follow-up

  • PhysicsNeMo #2205 replaces local compatibility code with the proposed FastGen loss hooks. It depends on PhysicsNeMo Add optional diffusion distillation strategy #2204 and FastGen [other] Add modulus.models.fcn_mip_plugin #44, not the other way around.

If FastGen maintainers decline FastGen #44, we would close PhysicsNeMo #2205.
The preceding PhysicsNeMo stack would remain available.

Review process

This draft replaces PhysicsNeMo #2098 and comes from the personal fork. This migration preserves the implementation. The first commit also includes the requested documentation clarifications.

Review the isolated commit diff for this layer. Each branch adds one commit to its prerequisite branch. All replacement PRs target upstream main, so later PRs include prerequisite commits awaiting merge in their Files changed tab. After prerequisites merge, update the remaining branches to remove them from those diffs.

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⏳ @CharlelieLrt — 56 file(s)

• docs/api/diffusion/distillation.rst
• docs/api/diffusion/introduction.rst
• docs/api/diffusion/models.rst
• docs/api/diffusion/noise_schedulers.rst
• docs/api/diffusion/preconditioners.rst
• physicsnemo/diffusion/init.py
• physicsnemo/diffusion/_fastgen.py
• physicsnemo/diffusion/base.py
• physicsnemo/diffusion/distillation/init.py
• physicsnemo/diffusion/distillation/_fastgen.py
• physicsnemo/diffusion/distillation/base.py
• physicsnemo/diffusion/distillation/checkpoint.py
• physicsnemo/diffusion/distillation/losses.py
• physicsnemo/diffusion/distillation/methods/init.py
• physicsnemo/diffusion/distillation/methods/dmd2/init.py
• physicsnemo/diffusion/distillation/methods/dmd2/_fastgen.py
• physicsnemo/diffusion/distillation/methods/f_distill/init.py
• physicsnemo/diffusion/distillation/methods/f_distill/_fastgen.py
• physicsnemo/diffusion/distillation/methods/meanflow/init.py
• physicsnemo/diffusion/distillation/methods/meanflow/_fastgen.py
• physicsnemo/diffusion/distillation/methods/scm/init.py
• physicsnemo/diffusion/distillation/methods/scm/_fastgen.py
• physicsnemo/diffusion/metrics/losses.py
• physicsnemo/diffusion/noise_schedulers/init.py
• physicsnemo/diffusion/noise_schedulers/_fastgen.py
• physicsnemo/diffusion/noise_schedulers/edm.py
• physicsnemo/diffusion/noise_schedulers/iddpm.py
• physicsnemo/diffusion/noise_schedulers/linear_gaussian.py
• physicsnemo/diffusion/noise_schedulers/rectified_flow.py
• physicsnemo/diffusion/noise_schedulers/trigflow.py
• physicsnemo/diffusion/noise_schedulers/ve.py
• physicsnemo/diffusion/noise_schedulers/vp.py
• physicsnemo/diffusion/preconditioners/init.py
• physicsnemo/diffusion/preconditioners/preconditioners.py
• physicsnemo/diffusion/utils/model_wrappers.py
• test/diffusion/data/distillation_meanflow_weighted.pth
• test/diffusion/data/distillation_scm_weighted.pth
• test/diffusion/data/trigflow_precond_conv.mdlus
• test/diffusion/data/trigflow_precond_conv_coefficients.pth
• test/diffusion/data/trigflow_precond_conv_forward.pth
• test/diffusion/data/trigflow_precond_conv_sigma.pth
• test/diffusion/data/trigflow_precond_linear.mdlus
• test/diffusion/data/trigflow_precond_linear_coefficients.pth
• test/diffusion/data/trigflow_precond_linear_forward.pth
• test/diffusion/data/trigflow_precond_linear_sigma.pth
• test/diffusion/helpers.py
• test/diffusion/test_distillation_api.py
• test/diffusion/test_distillation_checkpoint.py
• test/diffusion/test_distillation_losses.py
• test/diffusion/test_distribution_distillation.py
• test/diffusion/test_fastgen_network.py
• test/diffusion/test_fastgen_noise_scheduler.py
• test/diffusion/test_model_io.py
• test/diffusion/test_noise_schedulers.py
• test/diffusion/test_preconditioners.py
• test/diffusion/test_trigflow.py

⏳ @coreyjadams — 1 file(s)

• pyproject.toml

⏳ @ktangsali — 2 file(s)

• docs/api_diffusion.rst
• pyproject.toml

⏳ @megnvidia — 6 file(s)

• docs/api/diffusion/distillation.rst
• docs/api/diffusion/introduction.rst
• docs/api/diffusion/models.rst
• docs/api/diffusion/noise_schedulers.rst
• docs/api/diffusion/preconditioners.rst
• docs/api_diffusion.rst

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• CHANGELOG.md
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