[Transform] Add trafo to remove identity ops

src/finn/transformation/streamline/remove.py

+from finn.transformation import Transformation
+from finn.transformation.infer_shapes import InferShapes
+
+
+class RemoveIdentityOps(Transformation):
+    """Remove identity ops like Add/Sub with zero or Mul/Div with one"""
+
+    def apply(self, model):
+        graph = model.graph
+        node_ind = 0
+        graph_modified = False
+        for n in graph.node:
+            node_ind += 1
+            if (
+                n.op_type in ["Add", "Sub"]
+                and not model.is_fork_node(n)
+                and not model.is_join_node(n)
+            ):
+                A = model.get_initializer(n.input[1])
+                if A == 0:
+                    producer = model.find_producer(n.input[0])
+                    # remove node and wire output tensor to
+                    # output of producer node
+                    producer.output[0] = n.output[0]
+                    graph.node.remove(n)
+
+            elif (
+                n.op_type in ["Mul", "Div"]
+                and not model.is_fork_node(n)
+                and not model.is_join_node(n)
+            ):
+                A = model.get_initializer(n.input[1])
+                if A == 1:
+                    producer = model.find_producer(n.input[0])
+                    # remove node and wire output tensor to
+                    # output of producer node
+                    producer.output[0] = n.output[0]
+                    graph.node.remove(n)
+        model = model.transform(InferShapes())
+        return (model, graph_modified)