class_docstr: Image Transformer in mesh_tensorflow.
class_docstr_tok: ["Image","Transformer","in","mesh_tensorflow","."]
code: def pos_dim(self):
      return mtf.Dimension("pos", self._hparams.img_len)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import copy
  import mesh_tensorflow
  from tensor2tensor.layers import common_hparams
  from tensor2tensor.layers import common_layers
  from tensor2tensor.utils import mtf_model
  from tensor2tensor.utils import registry
  import tensorflow.compat.v1
  from tensorflow.compat.v1 import estimator
label_desc: Use image node_id 4 for calling the MtfImageTransformer obj's underlying member method code with example usage: obj.pos_dim() and returns: mtf