Skip labels before loop

[johnarevalo]

When there are many labels to be skipped, having this validation inside the for loop makes kbet computation significantly slow:

scib/scib/metrics/kbet.py

Lines 114 to 122 in ed3e284

	# subset by label
	adata_sub = adata_tmp[adata_tmp.obs[label_key] == clus, :].copy()
	# check if neighborhood size too small or only one batch in subset
	if np.logical_or(
	adata_sub.n_obs < 10, len(adata_sub.obs[batch_key].cat.categories) == 1
	):
	print(f"{clus} consists of a single batch or is too small. Skip.")
	score = np.nan

This PR extract such checks outside the for loop to do them in a single pass.