How to start from example diverging REST interface?
The attribute name that you should specify is the name of the variable that holds your FastAPI instance. As they say in the docs:

The ASGI application should be specified in the form path.to.module:instance.path.

In this case for you, it would be uvicorn main:app where main.py is the file your code is in and app is the name of the variable in that file that holds your FastAPI instance.

In the uvicorn exmaple, one writes uvicorn filename:attributename and by that start the server. However, the interface I have generated has no such method attributename in filename. Therefore, I am unsure what to pass as attributename.
Generated code in main.py
"""
    Somename

    Specification for REST-API of somename.

    The version of the OpenAPI document: 1.0.0
    Generated by: https://openapi-generator.tech
"""
from fastapi import FastAPI
from openapi_server.apis.some_api import router as SomeApiRouter

app = FastAPI(
    title="SomeName",
    description="Specification for REST-API of somename",
    version="1.0.0",
)

app.include_router(SomeApiRouter)

