class_docstr: Singleton builder.
  
  Example:
  
  from coremltools.converters.mil.mil import Builder as mb
  from coremltools.converters.mil.mil import Program, Function
  
  prog = Program()
  func_inputs = {"x": mb.placeholder(_shape=[2,3]),
                 "y": mb.placeholder(_shape=[2,3])}
  with Function(func_inputs) as ssa_fun:
    x, y = ssa_fun.inputs['x'], ssa_fun.inputs['x']
    res_var = mb.add(x=x, y=y) # created within ssa_fun block
    ssa_fun.set_outputs([res_var])
  prog.add_function("main", ssa_fun)
code: def placeholder(shape, dtype=None):
      return Placeholder(shape, dtype)
  
imports: from collections import defaultdict
  import copy
  import logging
  import six
  import numbers
  import numpy
  from coremltools.converters.mil.mil.types.symbolic import any_symbolic
  from .None import curr_block, Program, Function, Placeholder, is_internal_input
  from .input_type import _InputType, InternalStringInputType, InternalScalarOrTensorInputType, ScalarOrTensorInputType, TupleInputType, InputSpec, InternalInputType, PyFunctionInputType
  from .var import InternalVar, Var
  from .ops import defs
label_desc: Use image node_id 6 for calling the Builder obj's underlying member method code with example usage: obj.placeholder(shape, dtype) and returns: Placeholder