remove the hard-coded 3d constraint for resnet

[wyli]

Is your feature request related to a problem? Please describe.
the resnet implementation is hardcoded for 3d only while the network API support both 2d and 3d

MONAI/monai/networks/nets/resnet.py

Line 230 in 62425d7

assert spatial_dims == 3

all the underlying blocks should be reviewed and refined to support both 2d and 3d.

[yiheng-wang-nv]

Hi @Douwe-Spaanderman , could you please help for this task? I do not understand the parameter shortcut_type.