[Analysis] Added analysis pass for custom node verification

src/finn/analysis/fpgadataflow/verify_custom_nodes.py

+import finn.custom_op.registry as registry
+
+
+def verify_nodes(model):
+    """Checks if custom ops in graph are correctly built, with all attributes
+    and inputs. Returns {node op_type : info_messages}
+    *info_messages is list of strings about the result of the verification"""
+
+    verification_dict = {}
+    for node in model.graph.node:
+        if node.domain == "finn":
+            op_type = node.op_type
+            inst = registry.custom_op[op_type](node)
+            verification_dict[op_type] = inst.verify_node()
+
+    return verification_dict