Fix kg_emb broken import: SampleBaseDataset -> SampleDataset, add pan…

[userjuma]

Closes #952

Fixes the broken import in kg_emb caused by SampleBaseDataset being renamed to SampleDataset in PyHealth 2.0. Updated across all 7 affected files (2 in kg_emb/datasets, 5 in kg_emb/models).

Also adds pandarallel (>=1.6.5) to pyproject.toml. It is used in kg_emb/datasets/base_kg_dataset.py but was previously missing from declared dependencies, which caused a ModuleNotFoundError immediately after the dataset rename was resolved.

Added tests/core/test_kg_emb.py covering:
1.Clean import of the kg_emb module
2. Importability of all five kg_emb model classes
3.A regression check ensuring TransE's dataset parameter is explicitly typed as SampleDataset

Manual verification:
Reproduced the original crash via python -c "import pyhealth.medcode.pretrained_embeddings" and confirmed it now resolves cleanly end-to-end.

[AxelNoun]

Thanks for this. The ImportError diagnosis is right, and the failure chain in
your description, SampleBaseDataset first and then pandarallel, matches what
I reproduced exactly.

I think one step is missing though. SampleDataset in 2.0 isn't the successor
to SampleBaseDataset: it's a litdata.StreamingDataset that expects a
directory containing schema.pkl. SampleKGDataset.__init__ still passes a
list, so:

ValueError: dir_path must be either a string, Path, or Dir, got: <class 'list'>

The module imports, but set_task() can't return anything usable. The three
tests here don't catch it because none of them instantiates the class.

Two smaller notes:

• pandarallel: initialize() is indeed called in umls.py, but the import in
  base_kg_dataset.py, the one cited in the description, is dead, and there is
  no parallel_apply anywhere in kg_emb. It is also not declared in
  pyproject.toml on master, which is what produced the ModuleNotFoundError
  in the first place.
• The test comparing TransE.__init__'s annotation to SampleDataset locks in
  an incorrect hint: KGEBaseModel reads dataset.entity_num, which only
  exists on SampleKGDataset.

Worth saying that #952 asked for the rename across eight files and your PR does
exactly that, so this is really about how the issue was framed.

I've opened #1202 going after the root cause, and credited you in the
description. Would be glad to have your review on it.