Add layer_map argument to NameMapComposite.

This way it is possible to specify a fallback layer map if we want to apply more general rules to the remaining layers.
The parameter is optional. If None is passed, there's no change in functionality.
If a layer_map list is passed with the same format as for the LayerMapComposite, then this will be used as a fallback if there's no hook for this specific layer name.

src/zennit/composites.py

@@ -110,11 +110,14 @@ class NameMapComposite(Composite):
     ----------
     name_map: `list[tuple[tuple[str, ...], Hook]]`
         A mapping as a list of tuples, with a tuple of applicable module names and a Hook.
+    layer_map: list[tuple[tuple[torch.nn.Module, ...], Hook]], optional
+        A mapping as a list of tuples, with a tuple of applicable module types and a Hook.
     canonizers: list[:py:class:`zennit.canonizers.Canonizer`], optional
         List of canonizer instances to be applied before applying hooks.
     '''
-    def __init__(self, name_map, canonizers=None):
+    def __init__(self, name_map, layer_map=None, canonizers=None):
         self.name_map = name_map
+        self.layer_map = layer_map if layer_map is not None else ()
         super().__init__(module_map=self.mapping, canonizers=canonizers)
 
     # pylint: disable=unused-argument
@@ -133,9 +136,19 @@ def mapping(self, ctx, name, module):
         Returns
         -------
         obj:`Hook` or None
-            The hook found with the module type in the given name map, or None if no applicable hook was found.
+            The hook found with the module name in the given name map, or with
+            the module type in the given layer map, or None if no applicable
+            hook was found.
         '''
-        return next((hook for names, hook in self.name_map if name in names), None)
+        return next(
+            (hook for names, hook in self.name_map if name in names),
+            next(
+                (hook for types, hook in self.layer_map
+                 if isinstance(module, types)
+                ),
+                None,
+            ),
+        )
 
 
 COMPOSITES = {}