refactor: extract shared tabulate math, remove pt_expt→pt dependency (#5339)

## Summary

- Extract tabulation math (activation derivatives, chain-rule
propagation, embedding-net forward pass) from
`deepmd/pt/utils/tabulate.py` into a shared numpy module
`deepmd/utils/tabulate_math.py`
- Both pt and pt_expt backends now inherit from the shared `DPTabulate`
class
- pt_expt no longer imports from `deepmd.pt` for tabulation or
compression
- Remove `is_pt` flag from `BaseTabulate` — normalize lower/upper bounds
to per-type scalars, move `_convert_numpy_float_to_int` into subclass
`_convert_numpy_to_tensor`

## Changes

| File | Lines | What |
|------|-------|------|
| `deepmd/utils/tabulate_math.py` | +582 | New: numpy DPTabulate with
all math |
| `deepmd/pt/utils/tabulate.py` | -560 | Thin wrapper: _get_descrpt_type
+ _convert_numpy_to_tensor |
| `deepmd/pt_expt/utils/tabulate.py` | rewrite | Inherit from
tabulate_math, no pt imports |
| `deepmd/pt_expt/descriptor/*.py` | -6 files | Remove ActivationFn
import, pass string |
| `deepmd/utils/tabulate.py` | -12 | Remove is_pt flag and branching |
| `deepmd/tf/utils/tabulate.py` | -1 | Remove is_pt=False arg |

## Test plan

- [x] PT compression tests pass (16 tests: se_a, se_atten, se_r, se_t,
dpa2)
- [x] PT_EXPT model compression tests pass (6 tests)
- [x] PT_EXPT descriptor compression tests pass (16 tests)
- [x] No `deepmd.pt` imports in pt_expt tabulate or descriptors
- [x] No `is_pt` references remain

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

* **Refactor**
* Streamlined tabulation: removed backend-specific PT logic, normalized
activation/exclude handling, and standardized numpy→tensor conversion
into a thin runtime wrapper.

* **New Features**
* Added a backend‑neutral math tabulation component that provides
activation derivatives and chain‑rule propagation for embedding
computations.

* **Tests**
* Converted tests to NumPy inputs/outputs for backend independence and
added an explicit linear‑activation derivative test.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Han Wang <wang_han@iapcm.ac.cn>

Author: wanghan-iapcm

deepmd/dpmodel/utils/network.py

@@ -44,6 +44,18 @@ def sigmoid_t(x):  # noqa: ANN001, ANN201
     return xp_sigmoid(x)
 
 
+def softplus_t(x):  # noqa: ANN001, ANN201
+    """Numerically stable softplus."""
+    xp = array_api_compat.array_namespace(x)
+    positive = x > 0
+    exp_neg_abs = xp.exp(xp.where(positive, -x, x))
+    return xp.where(
+        positive,
+        x + xp.log(1 + exp_neg_abs),
+        xp.log(1 + exp_neg_abs),
+    )
+
+
 class Identity(NativeOP):
     def __init__(self) -> None:
         super().__init__()
@@ -330,9 +342,7 @@ def fn(x):  # noqa: ANN001, ANN202
     elif activation_function == "softplus":
 
         def fn(x):  # noqa: ANN001, ANN202
-            xp = array_api_compat.array_namespace(x)
-            # generated by GitHub Copilot
-            return xp.log(1 + xp.exp(x))
+            return softplus_t(x)
 
         return fn
     elif activation_function == "sigmoid":