code: def _unitary(norm):
      if norm not in (None, "ortho", "no_norm"):
          raise ValueError(
              "Invalid value %s for norm, must be None, 'ortho' or "
              "'no norm'" % norm
          )
      return norm
  
imports: from __future__ import absolute_import, print_function, division
  import numpy
  import theano
  from theano import Op
  import theano.tensor
  from theano.gradient import DisconnectedType
  from .basic_ops import gpu_contiguous, as_gpuarray_variable, infer_context_name
  from .type import GpuArrayType
  import theano.tensor.fft
  from .opt import register_opt, op_lifter, register_opt2
label_desc: Use image node_id 3 for calling a global function with example usage: _unitary(norm) and returns: norm