Fix ImportError in kg_emb caused by SampleBaseDataset rename

[MadhukarJukanti]

Fixes #952.

SampleBaseDataset was renamed to SampleDataset during the PyHealth 2.0
upgrade, but 7 files in the kg_emb module were never updated. Any user
importing from pyhealth.medcode.pretrained_embeddings.kg_emb hit an
immediate ImportError because SampleBaseDataset no longer exists in
pyhealth.datasets.

Updated all imports and type annotations across the affected files:

• datasets/sample_kg_dataset.py
• datasets/splitter.py
• models/kg_base.py
• models/complex.py
• models/distmult.py
• models/rotate.py
• models/transe.py

Test plan

• from pyhealth.medcode.pretrained_embeddings.kg_emb.datasets import SampleKGDataset no longer raises
  ImportError
• from pyhealth.medcode.pretrained_embeddings.kg_emb.models import ComplEx, DistMult, RotatE, TransE no
  longer raises ImportError

And a short commit message (no body needed since the PR description covers it):

fix: update kg_emb imports from SampleBaseDataset to SampleDataset