Composites: Cooperative LayerMapComposite

[chr5tphr]

Composites: Cooperative LayerMapComposite

• add the parameter layer_map to built-in subclasses of
  LayerMapComposite, which will be prepended to the composite's
  layer_map
• add the parameter layer_map and first_map to built-in subclasses
  of SpecialFirstLayerMapComposite, which will be prepended to the
  composites layer_map and first_map respectively
• this is done to let the user easily introduce small changes to the
  built-in composites, for example to add rules for missing layers, or
  change the behavior for existing modules
• the most important use-case for this is adding a mapping for MaxPool layers
  to use another rule without the need to implement a full composite
• for this use-case, also add the abstract type MaxPool

Tests: Cooperative LayerMapComposite

• added tests for built-in cooperative LayerMapComposites
• built-in composites now get cooperative variations for tests in
  conftest.py
• these variations get custom rules PassClone and GradClone
• the registered rule for each module is now checked only for the first
  match in the module_map, which would previously lead to errors when
  one module had more than one matching rule

Docs: Cooperative LayerMapComposite

• add documentation for cooperative LayerMapComposite subtypes
• add subsections to how-to/use-rules-composites-and-canonizers
  Composite