Composite: fix passing of beta_smooth value

The BetaSmooth composite did not pass the beta_smooth value to the ReLUBetaSmooth hook constructor resulting in beta_smooth==10 at all times.

src/zennit/composites.py

@@ -494,6 +494,6 @@ def __init__(self, beta_smooth=10., layer_map=None, zero_params=None, canonizers
             layer_map = []
 
         layer_map = layer_map + [
-            (torch.nn.ReLU, ReLUBetaSmooth()),
+            (torch.nn.ReLU, ReLUBetaSmooth(beta_smooth=beta_smooth)),
         ]
         super().__init__(layer_map=layer_map, canonizers=canonizers)