make type hints more specific than Any in _generate_step_dtypes(self)

[diegomarvid]

    # TODO(@palf): make type hints more specific than Any, however I don't know the return type of
    # the values in dtypes. [link to issue]
    @property
    def _generate_step_dtypes(self) -> dict[str, Any]:
        """
        Get the schema of the raw data produced by the GenerateStep during training.

        This schema is a dictionary mapping dataframe columns (after drops) to their observed types
        during training

        Returns
        -------
        dict[str, Any]
            A dictionary with columns in keys and their training types in values
        """
        return self["_generate_step_dtypes"]