docs(loss): document loss_func/f_use_norm and three-value huber_delta

[njzjz]

Parent issue: #5917 (checklist items 13–14)

Scope

Two energy-loss options exist only as schema entries:

1. Force L2-norm / MAE loss (3f52fa95a, feat(dp, pt): add force l2 norm loss & mae loss, PR feat(dp, pt): add force l2 norm loss & mae loss #5294). NOTE: the shipped config keys are loss_func ("mse" / "mae") and f_use_norm, not force_l2_norm — the original checklist item title used a pre-merge name. Schema: deepmd/utils/argcheck.py:4319-4331 (doc_loss_func, doc_f_use_norm), arguments at :4468-4479. Semantics: loss_func selects MSE vs MAE for energy/force/virial/atom losses; f_use_norm uses the L2 norm of force vectors in the loss and is only valid when loss_func="mae" or use_huber=True (validation enforced).
2. Three-value Huber delta (14c349b42): huber_delta accepts a single float or a list of three values (schema at argcheck.py:4481-4487, [float, list[float]], default 0.01). Verify from the loss implementation which three losses the three values correspond to (likely energy/force/virial — confirm in deepmd/dpmodel/loss/ener.py and deepmd/pt/loss/ener.py).

Neither appears in doc/model/train-energy.md; grep huber doc/ returns nothing.

Implementation plan

doc/model/train-energy.md

Read the loss section of the page plus argcheck.py entries and the DP/PT ener loss implementations first. Extend the loss prose (near the use_huber / pref documentation):

• loss_func: "mse" (verify default) vs "mae", which loss terms it applies to, backend support (DP & PyTorch per the commit title — verify TF support is absent before claiming);
• f_use_norm: what it changes for force loss (norm of the force vector instead of per-component), and the validity constraint (loss_func="mae" or use_huber=True);
• use_huber + huber_delta: Huber loss threshold; document scalar vs three-value forms and their meaning, with default 0.01 (verify per-backend support from code);
• A minimal JSON example showing loss_func: "mae" with f_use_norm: true, and one with a three-value huber_delta.

Constraints

• Use the shipped key names (loss_func, f_use_norm); the checklist's force_l2_norm was a working title.
• Verify defaults and backend support in argcheck.py and the loss classes; state backend restrictions explicitly.
• Keep math/style consistent with the surrounding loss documentation.

Verification

• grep -n "loss_func\|f_use_norm\|huber_delta" doc/model/train-energy.md shows the new prose.

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Parent issue: #5917

Coding agent: opencode
opencode version: 1.18.9
Model: ustc/k3
Reasoning effort: max