FIX: Replace Tags and Subjob stub structs

[maxnystrom]

For the initial release, these placeholders were added as these objects are undefined in the API documentation.

The task here is to determine the schema of these objects and update their corresponding structs.

together-go/fine_tuning.go

Line 45 in c60d6e1

type Tags struct{} // TODO: define Tags.

together-go/fine_tuning.go

Line 46 in c60d6e1

type Subjob struct{} // TODO: define Subjob.