fix(frontend): non fusable graph is not an error

reported in https://community.zama.ai/t/implementing-comparison-strategies/3469/5

Author: rudy-6-4

frontends/concrete-python/concrete/fhe/compilation/utils.py

@@ -248,12 +248,15 @@ def fuse(graph: Graph, artifacts: Optional["FunctionDebugArtifacts"] = None):
         all_nodes, start_nodes, terminal_node = subgraph_to_fuse
         processed_terminal_nodes.add(terminal_node)
 
-        conversion_result = convert_subgraph_to_subgraph_node(
-            graph,
-            all_nodes,
-            start_nodes,
-            terminal_node,
-        )
+        try:
+            conversion_result = convert_subgraph_to_subgraph_node(
+                graph,
+                all_nodes,
+                start_nodes,
+                terminal_node,
+            )
+        except NotFusable:
+            conversion_result = None
         if conversion_result is None:
             continue
 
@@ -769,7 +772,7 @@ def convert_subgraph_to_subgraph_node(
         if terminal_node.properties["name"] == "where":
             return None
 
-        raise RuntimeError(
+        raise NotFusable(
             "A subgraph within the function you are trying to compile cannot be fused "
             "because it has multiple input nodes\n\n"
             + graph.format(highlighted_nodes=base_highlighted_nodes, show_bounds=False)
@@ -826,6 +829,8 @@ def convert_subgraph_to_subgraph_node(
 
     return subgraph_node, variable_input_node
 
+class NotFusable(RuntimeError):
+    pass
 
 def check_subgraph_fusibility(
     graph: Graph,
@@ -868,7 +873,7 @@ def check_subgraph_fusibility(
 
         if not node.is_fusable:
             base_highlighted_nodes[node] = ["this node is not fusable", node.location]
-            raise RuntimeError(
+            raise NotFusable(
                 "A subgraph within the function you are trying to compile cannot be fused "
                 "because of a node, which is marked explicitly as non-fusable\n\n"
                 + graph.format(highlighted_nodes=base_highlighted_nodes, show_bounds=False)
@@ -879,7 +884,7 @@ def check_subgraph_fusibility(
                 "this node has a different shape than the input node",
                 node.location,
             ]
-            raise RuntimeError(
+            raise NotFusable(
                 "A subgraph within the function you are trying to compile cannot be fused "
                 "because of a node, which is has a different shape than the input node\n\n"
                 + graph.format(highlighted_nodes=base_highlighted_nodes, show_bounds=False)

frontends/concrete-python/tests/execution/test_comparison.py

@@ -64,6 +64,7 @@
                     ("<=", lambda x, y: x <= y),
                     (">", lambda x, y: x > y),
                     (">=", lambda x, y: x >= y),
+                    ("mixed", lambda x, y:  fhe.if_then_else(x < y, x, y))
                 ]
             ),
             # bit widths