code: def mt_group_by_memory_usage(mt_path):
      mt = hl.read_matrix_table(mt_path)
      mt = mt.group_rows_by(new_idx=mt.row_idx % 3).aggregate(
          x=hl.agg.mean(mt.x)
      )
      mt._force_count_rows()
  
imports: from os import path
  from tempfile import TemporaryDirectory, NamedTemporaryFile
  import hail
  from .resources import *
  from .utils import benchmark
label_desc: Use image node_id 42 for calling a global function with example usage: mt_group_by_memory_usage(mt_path) without return types