[Dist] partition_graph crash if no etyped edges or ntyped nodes are available

[Rhett-Ying]

🐛 Bug

To Reproduce

dgl/python/dgl/distributed/partition.py

Lines 1098 to 1106 in f71427f

	for etype in g.canonical_etypes:
	inner_etype_mask = inner_etype == g.get_etype_id(etype)
	typed_eids = np.sort(
	F.asnumpy(F.boolean_mask(inner_eids, inner_etype_mask))
	)
	assert np.all(
	typed_eids
	== np.arange(int(typed_eids[0]), int(typed_eids[-1]) + 1)
	)

Expected behavior

Environment

• DGL Version (e.g., 1.0):
• Backend Library & Version (e.g., PyTorch 0.4.1, MXNet/Gluon 1.3):
• OS (e.g., Linux):
• How you installed DGL (conda, pip, source):
• Build command you used (if compiling from source):
• Python version:
• CUDA/cuDNN version (if applicable):
• GPU models and configuration (e.g. V100):
• Any other relevant information:

Additional context