Make sure __init__ is called in graph callback. (#3881)

I made the mistake and got a segmentation fault.  A value error might be nicer.

Authors:
  - Jiaming Yuan (https://github.com/trivialfis)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #3881

python/cuml/internals/callbacks_implems.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2021, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -71,8 +71,8 @@ namespace ML {
                 }
 
             public:
-                PyObject* pyCallbackClass;
+                PyObject* pyCallbackClass = nullptr;
         };
 
     }
-}
+}

python/cuml/internals/internals.pyx

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2019, NVIDIA CORPORATION.
+# Copyright (c) 2019-2021, NVIDIA CORPORATION.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -79,4 +79,8 @@ cdef class GraphBasedDimRedCallback(PyCallback):
         self.native_callback.pyCallbackClass = <PyObject *><void*>self
 
     def get_native_callback(self):
+        if self.native_callback.pyCallbackClass == NULL:
+            raise ValueError(
+                "You need to call `super().__init__` in your callback."
+            )
         return <uintptr_t>&(self.native_callback)

python/cuml/test/test_internals_api.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2018-2019, NVIDIA CORPORATION.
+# Copyright (c) 2018-2021, NVIDIA CORPORATION.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -24,6 +24,10 @@
 class CustomCallback(GraphBasedDimRedCallback):
     preprocess_event, epoch_event, train_event = False, 0, False
 
+    def __init__(self, skip_init=False):
+        if not skip_init:
+            super().__init__()
+
     def check(self):
         assert(self.preprocess_event)
         assert(self.epoch_event > 10)
@@ -45,3 +49,10 @@ def test_internals_api(n_components):
     reducer = UMAP(n_components=n_components, callback=callback)
     reducer.fit(data)
     callback.check()
+
+    # Make sure super().__init__ is called
+    callback = CustomCallback(skip_init=True)
+    model = UMAP(n_epochs=10, callback=callback)
+
+    with pytest.raises(ValueError):
+        model.fit_transform(data)