feat(dpmodel): backend-agnostic DPA4 descriptor and fitting (core path)

deepmd/dpmodel/descriptor/__init__.py

@@ -8,6 +8,9 @@
 from .dpa3 import (
     DescrptDPA3,
 )
+from .dpa4 import (
+    DescrptDPA4,
+)
 from .hybrid import (
     DescrptHybrid,
 )
@@ -34,6 +37,7 @@
     "DescrptDPA1",
     "DescrptDPA2",
     "DescrptDPA3",
+    "DescrptDPA4",
     "DescrptHybrid",
     "DescrptSeA",
     "DescrptSeAttenV2",