## What

Adds the DPA4/SeZM `so3_readout` option (`"none"` / `"glu"` / `"mlp"`)
**across all backends** (pt + dpmodel + pt_expt), making it
cross-backend consistent.

Builds on **#5556** (@OutisLi): its pt `so3_readout` commit
(`refactor(dpa4): output ffn`) is included here with original authorship
preserved; this PR adds the missing **dpmodel** counterpart so the
shared DPA4 serialize format round-trips across backends. (The unrelated
`nv_nlist`/compiler-check fixes from #5556 are intentionally left to
#5556.)

## Why

`so3_readout` is implemented by configuring the final output FFN —
`"glu"`/`"mlp"` turn on the SO(3)-grid FFN (`ffn_so3_grid`, `grid_mlp`).
On its own (pt-only, as in #5556) it breaks DPA4 cross-backend
consistency: pt `serialize()` emits `so3_readout` but dpmodel
`DescrptDPA4` couldn't round-trip it →
`source/tests/consistent/descriptor/test_dpa4.py::...::test_pt_consistent_with_ref`
failed on every Test Python shard.

This is now feasible and small because **#5555** already ported
`ffn_so3_grid` + the SO(3)-grid machinery
(`SO3GridNet`/`GridMLP`/`GridProduct`) into the dpmodel
`EquivariantFFN`. So the dpmodel `so3_readout` is just: accept the
param, configure `output_ffn` exactly like pt, wire the readout (l=0
slice for `"none"`; full `(N,D,1,C)` fold for `"glu"`/`"mlp"` then slice
l=0), and serialize the key. pt_expt auto-wraps.

## Changes

- **pt** (from #5556): `so3_readout` in `DescrptSeZM` + argcheck +
`examples/water/dpa4/input.json`.
- **dpmodel** `descriptor/dpa4.py`: `so3_readout` param + validation;
`output_ffn` configured (`lmax=node_l_schedule[-1]`, `kmax=min(kmax,
readout_lmax)`, `ffn_so3_grid`, `grid_mlp`, `grid_branch=0`); readout
forward mirrors pt; serialize the key.
- **pt_expt**: auto-wrapped (no explicit change).

## Validation

- `test_dpa4.py` cross-backend consistency rows for `so3_readout ∈
{none, glu, mlp}` (pt vs dpmodel vs pt_expt, mixed_types) — green;
`test_pt_consistent_with_ref` now passes.
- Full-descriptor pt→dpmodel parity (weight-copied, `glu`+`mlp`) —
**~7e-15 abs** (gate 1e-10), proving serialize interop.
- dpa4 suite: 611 passed; ruff clean.

## Notes

- Depends on #5555 (merged) for the dpmodel SO(3)-grid FFN.
- `so3_readout` no longer "(Supported Backend: PyTorch)" — now
multi-backend.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

## Release Notes

* **New Features**
* Added a configurable `so3_readout` option to the DPA4 and SeZM
descriptors (modes: `"none"`, `"glu"`, `"mlp"`), controlling how the
final SO(3) readout is computed.
* The setting is included in descriptor configuration
serialization/deserialization to support round-tripping.
  * Updated the water DPA4 example to use `so3_readout: "mlp"`.

* **Tests**
* Added tests covering multiple readout modes, backend parity between
implementations, and correct behavior for edge-free scenarios.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: OutisLi <LTC201806070316@gmail.com>
Co-authored-by: Han Wang <wang_han@iapcm.ac.cn>