[Unity][Transform] Raise error in FuseOpsByPattern for SSA violation (#16421)

Internally, `FuseOpsByPattern` makes a mapping from relax variables to
the fused group containing that variable.  If the input module
violates SSA, this map may be ill-formed.  While not strictly
necessary for FuseOps to handle ill-formed inputs, checking it at this
level provides better error handling than propagating it to downstream
passes.

This commit checks for ill-formed inputs that would produce invalid
fused outputs and raises an error.

Author: Lunderberg

src/relax/transform/fuse_ops.cc

@@ -1286,7 +1286,14 @@ IRModule FuseOpsByPattern(const tvm::Array<transform::FusionPattern>& patterns,
                                               pattern->annotation_patterns,
                                               pattern->check.value_or(nullptr), entry.second,
                                               &arena, pattern->attrs_getter.value_or(nullptr));
-      group_map.insert(map.begin(), map.end());
+      for (const auto& [key, value] : map) {
+        CHECK(!group_map.count(key))
+            << "ValueError: "
+            << "IRModule is invalid.  "
+            << "The object " << GetRef<ObjectRef>(key) << " appears in multiple partitions, "
+            << "which can occur when the IRModule was not single-site assignment";
+        group_map.insert({key, value});
+      }
     }
     mod = MakeGroupedFunctions(mod, group_map, /*lift_constants*/ !bind_constants);
   }

tests/python/relax/test_transform_fuse_ops_by_pattern.py

@@ -1109,5 +1109,26 @@ def test_multple_runs():
     )
 
 
+@pytest.mark.skip_well_formed_check_before_transform
+def test_error_on_repeated_variable_definitions():
+    """Raise error for SSA violations
+
+    Internally, `FuseOpsByPattern` makes a mapping from relax
+    variables to the fused group containing that variable.  If the
+    input module violates SSA, this map may be ill-formed.
+
+    While not strictly necessary for FuseOps to handle ill-formed
+    inputs, checking it at this level provides better error handling
+    than propagating it to downstream passes.
+    """
+    mod = Conv2dReLU.clone()
+    mod["copy"] = mod["main"].with_attr("global_symbol", "copy")
+
+    patterns = [("dnnl.conv2d_relu", conv2d_relu_pat)]
+
+    with pytest.raises(ValueError):
+        relax.transform.FuseOpsByPattern(patterns)(mod)
+
+
 if __name__ == "__main__":
     pytest.main([__file__])