def __init__(self, inputs, opset_version=None):
        """
        inputs: str -> placeholder
        opset_version: AvailableTarget enum. Describes the opset version of the function
        """
        self.placeholder_inputs = inputs
        self.opset_version = opset_version
        self.output_types = None
        self.input_types = []

        # str -> Var
        self._input_dict = OrderedDict()
        for k, v in self.placeholder_inputs.items():
            v.set_name(k)  # set to user input name
            self._input_dict[k] = v.outputs[0]

        global k_used_symbols
        global k_num_internal_syms
        for inp in self._input_dict.values():
            if types.is_tensor(inp.dtype):
                shapes = inp.dtype.get_shape()
                for s in shapes:
                    if is_symbolic(s):
                        k_used_symbols.add(s)
        super().__init__()

        inputs: str -> placeholder
        opset_version: AvailableTarget enum. Describes the opset version of the function
        
coremltools/converters/mil/mil/block.py