Add workaround for PyNNDescentTransformer pickling problems

Author: Frankie Robertson

sklearn_ann/kneighbors/pynndescent.py

@@ -1,4 +1,15 @@
-from pynndescent import PyNNDescentTransformer
+from pynndescent import PyNNDescentTransformer as PyNNDescentTransformerBase
+
+
+def no_op():
+    pass
+
+
+class PyNNDescentTransformer(PyNNDescentTransformerBase):
+    def fit(self, X, compress_index=True):
+        super().fit(X, compress_index=compress_index)
+        self.index_.compress_index = no_op
+        return self
 
 
 __all__ = ["PyNNDescentTransformer"]