text: Title: Parafac: set fixed_modes default to None
  Body: In parafac, currently `fixed_modes` defaults to an empty list, which is mutable and mutations of that list would be preserved from one call to another. Set the default to None and initialize accordingly in the core of the function. 
  
  https://github.com/tensorly/tensorly/blob/ab504452039c3fe4f46351a622dd177b29778bd2/tensorly/decomposition/_cp.py#L176