Builds the Keras loss function corresponding to the object received

# S3 method for ruta_contraction
to_keras(x, keras_model, ...)

## Arguments

x |
A `"ruta_contraction"` object |

keras_model |
The keras autoencoder which will use the loss function |

... |
Rest of parameters, ignored |

## Value

A function which returns the contractive loss for given true and
predicted values