Small issue in atom2valence dictionary iteration in `Molecule.is_valid`

[UmarZein]

specs:
Python 3.11.4 (i am using my fork of torchdrug only to alleviate versioning constraints)
torch.__version__: 2.1.0+cu118
torchdrug.__version__: 0.2.1

---

calling Molecule.is_valid gives me the following error:

File ~\anaconda3\Lib\site-packages\torchdrug\data\molecule.py:521, in Molecule.is_valid(self)
    519 # TODO: cross-check by any domain expert
    520 atom2valence = torch.tensor(float("nan")).repeat(constant.NUM_ATOM)
--> 521 for k, v in self.atom2valence:
    522     atom2valence[k] = v
    523 atom2valence = torch.as_tensor(atom2valence, device=self.device)

TypeError: cannot unpack non-iterable int object

Wouldn't using dict.items() be the correct way to iterate over key-value pairs in a dict, as it is the case in PackerMolecule.is_valid?

for k,v in self.atom2valence.items():