code: def test_auto_n(self, xp, scp):
      # Test that our value of n is a reasonable choice (depends on
      # the downsampling factor)
      sfreq = 100.0
      n = 1000
      t = xp.arange(n) / sfreq
      # will alias for decimations (>= 15)
      x = xp.sqrt(2.0 / n) * xp.sin(2 * xp.pi * (sfreq / 30.0) * t)
      x_out = scp.signal.decimate(x, 30, ftype="fir")
      return x_out
  
imports: import cupy
  from cupy import testing
  import cupyx.scipy.signal
  import cupyx.scipy.signal.windows
  import numpy
  import pytest
  from math import gcd
label_desc: Use image node_id 10 for calling the TestDecimate obj's underlying member method code with example usage: obj.test_auto_n(xp, scp) and returns: x_out