`SplitDim` support chns==1

[wyli]

the error condition here is too restrictive:

MONAI/monai/transforms/utility/array.py

Lines 374 to 375 in ab800d8

	if n_out <= 1:
	raise RuntimeError(f"Input image is singleton along dimension to be split, got shape {img.shape}.")

e.g. single channel input works fine in numpy (and pytorch):

>>> a = np.zeros((1, 256, 256))
>>> s = np.split(a, 1, 0)
>>> len(s)
1

cc @holgerroth