Feat: set force label optional (#772)

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

- **Improvements**
- Enhanced flexibility in data handling by making forces data optional
in the system configuration.
- Added a method to check for the presence of forces data in the system.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Han Wang <[email protected]>

dpdata/system.py

@@ -1209,7 +1209,11 @@ class LabeledSystem(System):
     DTYPES: tuple[DataType, ...] = System.DTYPES + (
         DataType("energies", np.ndarray, (Axis.NFRAMES,), deepmd_name="energy"),
         DataType(
-            "forces", np.ndarray, (Axis.NFRAMES, Axis.NATOMS, 3), deepmd_name="force"
+            "forces",
+            np.ndarray,
+            (Axis.NFRAMES, Axis.NATOMS, 3),
+            required=False,
+            deepmd_name="force",
         ),
         DataType(
             "virials",
@@ -1269,13 +1273,17 @@ def __add__(self, others):
             raise RuntimeError("Unspported data structure")
         return self.__class__.from_dict({"data": self_copy.data})
 
+    def has_forces(self) -> bool:
+        return "forces" in self.data
+
     def has_virial(self) -> bool:
         # return ('virials' in self.data) and (len(self.data['virials']) > 0)
         return "virials" in self.data
 
     def affine_map_fv(self, trans, f_idx: int | numbers.Integral):
         assert np.linalg.det(trans) != 0
-        self.data["forces"][f_idx] = np.matmul(self.data["forces"][f_idx], trans)
+        if self.has_forces():
+            self.data["forces"][f_idx] = np.matmul(self.data["forces"][f_idx], trans)
         if self.has_virial():
             self.data["virials"][f_idx] = np.matmul(
                 trans.T, np.matmul(self.data["virials"][f_idx], trans)
@@ -1308,7 +1316,8 @@ def correction(self, hl_sys: LabeledSystem) -> LabeledSystem:
             raise RuntimeError("high_sys should be LabeledSystem")
         corrected_sys = self.copy()
         corrected_sys.data["energies"] = hl_sys.data["energies"] - self.data["energies"]
-        corrected_sys.data["forces"] = hl_sys.data["forces"] - self.data["forces"]
+        if "forces" in self.data and "forces" in hl_sys.data:
+            corrected_sys.data["forces"] = hl_sys.data["forces"] - self.data["forces"]
         if "virials" in self.data and "virials" in hl_sys.data:
             corrected_sys.data["virials"] = (
                 hl_sys.data["virials"] - self.data["virials"]