fix: fix nopbc in dpdata driver (#4027)

Sometimes dpdata data may contain `"nopbc": False` (from ase or n2p2).

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## Summary by CodeRabbit

- **Bug Fixes**
- Improved handling of the "nopbc" key in the data processing logic,
enhancing the accuracy of cell variable assignments.


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Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

Author: njzjz

deepmd/driver.py

@@ -62,7 +62,8 @@ def label(self, data: dict) -> dict:
         atype = sorted_data["atom_types"]
 
         coord = data["coords"].reshape((nframes, natoms * 3))
-        if "nopbc" not in data:
+        # sometimes data["nopbc"] may be False
+        if not data.get("nopbc", False):
             cell = data["cells"].reshape((nframes, 9))
         else:
             cell = None