code: def fftfreq(n, d=1.0, gpu=None, chunk_size=None):
      """
      Return the Discrete Fourier Transform sample frequencies.
  
      The returned float tensor `f` contains the frequency bin centers in cycles
      per unit of the sample spacing (with zero at the start).  For instance, if
      the sample spacing is in seconds, then the frequency unit is cycles/second.
  
      Given a window length `n` and a sample spacing `d`::
  
        f = [0, 1, ...,   n/2-1,     -n/2, ..., -1] / (d*n)   if n is even
        f = [0, 1, ..., (n-1)/2, -(n-1)/2, ..., -1] / (d*n)   if n is odd
  
      Parameters
      ----------
      n : int
          Window length.
      d : scalar, optional
          Sample spacing (inverse of the sampling rate). Defaults to 1.
      gpu : bool, optional
          Allocate the tensor on GPU if True, False as default
      chunk_size : int or tuple of int or tuple of ints, optional
          Desired chunk size on each dimension
  
      Returns
      -------
      f : Tensor
          Array of length `n` containing the sample frequencies.
  
      Examples
      --------
      >>> import mars.tensor as mt
  
      >>> signal = mt.array([-2, 8, 6, 4, 1, 0, 3, 5], dtype=float)
      >>> fourier = mt.fft.fft(signal)
      >>> n = signal.size
      >>> timestep = 0.1
      >>> freq = mt.fft.fftfreq(n, d=timestep)
      >>> freq.execute()
      array([ 0.  ,  1.25,  2.5 ,  3.75, -5.  , -3.75, -2.5 , -1.25])
  
      """
      n, d = int(n), float(d)
      op = TensorFFTFreq(n=n, d=d, dtype=np.dtype(float), gpu=gpu)
      return op(chunk_size)
  
imports: import numpy
  from ...None import opcodes
  from ...core import NotSupportTile, recursive_tile
  from ...serialization.serializables import Int32Field, Float64Field, KeyField
  from ..operands import TensorOperand, TensorHasInput, TensorOperandMixin
  from ..datasource import arange
  from ..core import TensorOrder
label_desc: Use image node_id 1 for calling a global function with example usage: fftfreq(n, d, gpu, chunk_size) and returns: op