Constructs a transformer from an arbitrary callable.
A FunctionTransformer forwards its X (and optionally y) arguments to a
user-defined function or function object and returns the result of this
function. This is useful for stateless transformations such as taking the
log of frequencies, doing custom scaling, etc.
A FunctionTransformer will not do any checks on its function's output.
Note: If a lambda is used as the function, then the resulting
transformer will not be pickleable.
.. versionadded:: 0.17