I have created some gcp instances using terraform module:

```
module "instance_template" {
  source = "terraform-google modules/vm/google//modules/instance_template"
...
}
module "compute_instance" {
  source             = "terraform-google- 
   modules/vm/google//modules/compute_instance"
  num_instances      = 4
  ...
}

```

then how do I get and output the private ip of these 4 instances after I run terraform apply?