code: def img_3d_ones_mni():
      """Return a ones-filled 3D Nifti1Image (identity affine)."""
      return _img_3d_ones(
          shape=_shape_3d_default(), affine=_affine_mni()
      )
  
imports: import warnings
  import nibabel
  import numpy
  import pytest
  from nibabel import Nifti1Image
  from nilearn import image
  from nilearn.datasets.tests._testing import request_mocker
  from nilearn.datasets.tests._testing import temp_nilearn_data_dir
  from nilearn.experimental.surface.tests.conftest import make_mini_img, mini_img, mini_mesh
label_desc: Use image node_id 27 for calling a global function with example usage: img_3d_ones_mni() and returns: _img_3d_ones