Constructs, populates and returns a form.

def init_form(self, form_data, widget_attrs=None, template=None):
        """Constructs, populates and returns a form."""
        form = self.form(data=form_data)
        form.template = template
        # Attach flow attribute to have access from flow forms (usually to call get_arg_or_attr())
        form.flow = self
        if widget_attrs is not None:
            set_form_widgets_attrs(form, widget_attrs)
        return form