add sigmoid and softplus nonlinearities

Author: cw-tan

nequip/nn/mlp.py

@@ -22,6 +22,9 @@
     "gelu": torch.nn.GELU,
     "ssp": ShiftedSoftplus,
     "tanh": torch.nn.Tanh,
+    # not 0 -> 0
+    "sigmoid": torch.nn.Sigmoid,
+    "softplus": torch.nn.Softplus,
 }