Returns a function that applies the given fn on the input_feature.

fn the function to apply.
input_feature the name of the feature on which fn should be applied. If a nested feature needs to be transformed, then specify it using slashes. For example, to transform feature b inside a, use a/b.
output_feature the name of the feature where the result should be stored. If None, then input_feature will be overridden.

a function that applies fn to input_feature field of examples and stores the result in output_feature.