Funnel ReLU (FReLU) (#556)

* fix #541 #542

* Update train.py

* Add Frelu

* Update activations.py

PEP8 and format updates for commonality with models.common.Conv()

* Update activations.py

Update case

* Update activations.py

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

Author: AlexWang1900

utils/activations.py

@@ -61,3 +61,16 @@ class Mish(nn.Module):  # https://github.com/digantamisra98/Mish
     @staticmethod
     def forward(x):
         return x * F.softplus(x).tanh()
+        
+        
+# FReLU https://arxiv.org/abs/2007.11824 --------------------------------------
+class FReLU(nn.Module):
+    def __init__(self, c1, k=3):  # ch_in, kernel
+        super().__init()__()
+        self.conv = nn.Conv2d(c1, c1, k, 1, 1, groups=c1)
+        self.bn = nn.BatchNorm2d(c1)
+        
+    @staticmethod
+    def forward(self, x):
+        return torch.max(x, self.bn(self.conv(x)))
+