Add memory efficient Swish impl

Author: rwightman

timm/models/gen_efficientnet.py

@@ -371,11 +371,30 @@ def _decode_arch_def(arch_def, depth_multiplier=1.0, depth_trunc='ceil'):
     return arch_args
 
 
-def swish(x, inplace=False):
-    if inplace:
-        return x.mul_(x.sigmoid())
-    else:
-        return x * x.sigmoid()
+_USE_SWISH_OPT = True
+if _USE_SWISH_OPT:
+    class SwishAutoFn(torch.autograd.Function):
+        """ Memory Efficient Swish
+        From: https://blog.ceshine.net/post/pytorch-memory-swish/
+        """
+        @staticmethod
+        def forward(ctx, x):
+            result = x.mul(torch.sigmoid(x))
+            ctx.save_for_backward(x)
+            return result
+
+        @staticmethod
+        def backward(ctx, grad_output):
+            x = ctx.saved_variables[0]
+            sigmoid_x = torch.sigmoid(x)
+            return grad_output * (sigmoid_x * (1 + x * (1 - sigmoid_x)))
+
+
+    def swish(x, inplace=False):
+        return SwishAutoFn.apply(x)
+else:
+    def swish(x, inplace=False):
+        return x.mul_(x.sigmoid()) if inplace else x.mul(x.sigmoid())
 
 
 def sigmoid(x, inplace=False):