Add composable distillation loss hooks

[CharlelieLrt]

Description

This PR adds optional loss hooks for users who integrate FastGen into an external
training loop. Shared reductions support masking and weighting for sCM, MeanFlow,
DSM, VSD, and adversarial losses. DMD2 and f-distill callers can select a training
phase explicitly, and ratio helpers accept caller-owned bins and a process group:

losses, outputs = model.single_train_step(batch, iteration, phase="auxiliary")
h, bins = f_divergence_weights(
    scores, t, weighting_fn, bins=bins,
    logit_weight=score_weight, statistics_group=data_group,
)

Existing calls keep their defaults, and the external loop continues to own
optimizers and checkpoints. TRAINING_INTEGRATION.md describes the masking
contracts and extension points. PhysicsNeMo can then delegate shared loss math
to FastGen instead of maintaining private compatibility reductions.

Dependencies

• Depends on

  • This PR starts from FastGen main and has no prerequisite PR.

• Depends on this PR

  • PhysicsNeMo #2100 replaces local compatibility code with the proposed FastGen loss hooks. It depends on acceptance of these hooks.

• Independent PhysicsNeMo stack

  • PhysicsNeMo #2094 adds named diffusion inputs and outputs.
  • PhysicsNeMo #2095 adds inverse SNR and TrigFlow components.
  • PhysicsNeMo #2096 adds private adapters and public sCM and MeanFlow losses.
  • PhysicsNeMo #2097 extends the loss API to DMD2 and f-distill.
  • PhysicsNeMo #2098 adds checkpoint bundles for restarting distillation.
  • PhysicsNeMo #2099 adds the optional training strategy and the grouped changelog entry.

• Independent packaging proposal

  • FastGen #43 allows the NumPy 2 dependency used by PhysicsNeMo. It does not depend on these hooks.

If FastGen maintainers decline FastGen #44, we would close PhysicsNeMo #2100.
The preceding PhysicsNeMo stack would remain available.

Review process

Reviewers can compare this PR with its immediate parent. After a parent merges,
we update the remaining branches and their PR bases to keep each diff focused.