GENERATED_SQL in Field does not work as intended (at all)

Describe the bug
I am trying to create a field that has auto increment but is not a pk. I am trying to create an Object like IntField with the adjusted GENERATED_SQL variable but I don't get the expected behaviour.

To Reproduce

class MyField(Field, int):
    """
    Integer field.
    """
    SQL_TYPE = "INT"
    allows_generated = True

    def __init__(self, **kwargs: Any) -> None:
        kwargs["generated"] = bool(kwargs.get("generated", True))
        super().__init__(**kwargs)

    @property
    def constraints(self) -> dict:
        return {
            "ge": 1,
            "le": 9,
        }

    class _db_postgres:
        GENERATED_SQL = "SERIAL NOT NULL"

    class _db_sqlite:
        GENERATED_SQL = "INTEGER AUTOINCREMENT NOT NULL"

    class _db_mysql:
        GENERATED_SQL = "INT NOT NULL AUTO_INCREMENT"

In a model then I define

newfield = MyField(generated=True)

Using aerich to generate a migration I get:

...
"newfield" INT NOT NULL,
...

Expected behavior

...
"newfield" SERIAL NOT NULL,
...

Additional context
Changing SQL_TYPE solves the problem (e.g. to SERIAL) but was not my first idea since there is a generate keyword.
It seems that the GENERATED_SQL only kicks in if it is a primary key.

Reproduce the custom MyField example and generate a migration with aerich, then trace how Field.GENERATED_SQL is selected for non-primary-key fields across PostgreSQL, SQLite, and MySQL. Done means the generated migration uses the backend-specific GENERATED_SQL value, such as SERIAL NOT NULL for PostgreSQL, while preserving existing primary-key behavior.