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The example build command to build the GEMM operators into a library is here:
https://github.com/apache/incubator-mxnet/blob/master/example/extensions/lib_custom_op/Makefile#L21
The example python code to load the library and test the operator for both symbol and ndarray APIs is here:
https://github.com/apache/incubator-mxnet/blob/master/example/extensions/lib_custom_op/test_gemm.py