Fix SiLU breakage in RegNets for PT < 1.7 (#725)

Summary:
Pull Request resolved: #725

SiLU is only available from PT >= 1.7

Now that our tests work fine, we can finally detect and fix issues like these

Reviewed By: kazhang

Differential Revision: D27236649

fbshipit-source-id: ed5bc81a40d1a21def13d81b83b181c55e86f6f5

Author: mannatsingh

classy_vision/models/regnet.py

@@ -447,17 +447,15 @@ class RegNet(ClassyModel):
     def __init__(self, params: RegNetParams):
         super().__init__()
 
-        if params.activation_type == ActivationType.SILU and get_torch_version() < [
-            1,
-            7,
-        ]:
-            raise RuntimeError("SiLU activation is only supported since PyTorch 1.7")
-
+        silu = None if get_torch_version() < [1, 7] else nn.SiLU()
         activation = {
             ActivationType.RELU: nn.ReLU(params.relu_in_place),
-            ActivationType.SILU: nn.SiLU(),
+            ActivationType.SILU: silu,
         }[params.activation_type]
 
+        if activation is None:
+            raise RuntimeError("SiLU activation is only supported since PyTorch 1.7")
+
         # Ad hoc stem
         self.stem = {
             StemType.RES_STEM_CIFAR: ResStemCifar,