add "update_by_batch" in gene;

add flatten_conns as an inverse function for unflatten_conns;
add "test_flatten.ipynb" as test for them.

tensorneat/algorithm/neat/gene/base.py

@@ -32,6 +32,9 @@ class BaseGene:
     def forward(self, state, attrs, inputs):
         raise NotImplementedError
 
+    def update_by_batch(self, state, attrs, batch_inputs):
+        raise NotImplementedError
+
     @property
     def length(self):
         return len(self.fixed_attrs) + len(self.custom_attrs)