human
Complete the below
def custom_model(*args, **kwargs):

    ''''Create a model from a user defined function. The inputs and parameters of
    the model will be inferred from the arguments of the function.
    This can be used either as a function or as a decorator.  See below for
    examples of both usages.
    .. note::
    All model parameters have to be defined as keyword arguments with
    default values in the model function.  Use `None` as a default argument
    value if you do not want to have a default value for that parameter.
    Parameters
    func : function
    Function which defines the model.  It should take N positional
    arguments where ``N`` is dimensions of the model (the number of
    independent variable in the model), and any number of keyword arguments
    (the parameters).  It must return the value of the model (typically as
    an array, but can also be a scalar for scalar inputs).  This
    corresponds to the `~astropy.modeling.Model.evaluate` method.
    fit_deriv : function, optional
    Function which defines the Jacobian derivative of the model. I.e., the
    derivative with respect to the *parameters* of the model.  It should
    have the same argument signature as ``func``, but should return a
    sequence where each element of the sequence is the derivative
    with respect to the corresponding argument. This corresponds to the
    :meth:`~astropy.modeling.FittableModel.fit_deriv` method.
    Examples
    Define a sinusoidal model function as a custom 1D model::
    >>> from astropy.modeling.models import custom_model
    >>> import numpy as np
    >>> def sine_model(x, amplitude=1., frequency=1.):
    ...     return amplitude * np.sin(2 * np.pi * frequency * x)
    >>> def sine_deriv(x, amplitude=1., frequency=1.):
    ...     return 2 * np.pi * amplitude * np.cos(2 * np.pi * frequency * x)
    >>> SineModel = custom_model(sine_model, fit_deriv=sine_deriv)
    Create an instance of the custom model and evaluate it::
    >>> model = SineModel()
    >>> model(0.25)
    1.0
    This model instance can now be used like a usual astropy...'''
 ... Give me a the Python program you requested?
assistant
#Here is the Python program you requested.
    fit_deriv = kwargs.get(u'fit_deriv', None)
    if ((len(args) == 1) and six.callable(args[0])):
        return _custom_model_wrapper(args[0], fit_deriv=fit_deriv)
    elif (not args):
        return functools.partial(_custom_model_wrapper, fit_deriv=fit_deriv)
    else:
        raise TypeError(u'{0}    takes    at    most    one    positional    argument    (the    callable/function    to    be    turned    into    a    model.        When    used    as    a    decorator    it    should    be    passed    keyword    arguments    only    (if    any).'.format(__name__))