train_mlp_test.rs: create a folder to which a model schema is output

[Surpris]

This library does not create the folder to which any model schema is output after users run train_mlp_test.rs.
This leads to an error that the main() in train_mlp_test.rs cannot output the model schema due to the non-existence of the out folder.

[Surpris]

This issue should be solved after issue #11 since this library is not up to date