There is no such code; Heat stores the template used to create/update a stack, and you can retrieve it again from the API, but it doesn't generate templates from stacks.

Heat-api client side controller:

StackController.generate_template() @ heat/api/openstack/v1/stacks.py is the one receives API request and send it to heat engine via RPC message.

Heat-engine server side implementation:

EngineService.generate_template() in heat/engine/service.py

This is basically to generate a template, whereas if you need to know about template_show functionality against a particular stack then refer get_template() methods at both the ends.