Training with a custom service account needs a new field named service account
trainingInput:
  serviceAccount: CUSTOM_SERVICE_ACCOUNT

https://cloud.google.com/ai-platform/training/docs/custom-service-account


airflow/providers/google/cloud/operators/mlengine.py
:param labels: a dictionary containing labels for the job; passed to BigQuery
    :type labels: Dict[str, str]
    :param impersonation_chain: Optional service account to impersonate using short-term
        credentials, or chained list of accounts required to get the access_token
        of the last account in the list, which will be impersonated in the request.
        If set as a string, the account must grant the originating account
        the Service Account Token Creator IAM role.
        If set as a sequence, the identities from the list must grant
        Service Account Token Creator IAM role to the directly preceding identity, with first
        account from the list granting this role to the originating account (templated).
    :type impersonation_chain: Union[str, Sequence[str]]
    """
    template_fields = [
        '_project_id',
        '_job_id',
        '_package_uris',
        '_training_python_module',
        '_training_args',
        '_region',
        '_scale_tier',
        '_master_type',
        '_master_config',
        '_runtime_version',
        '_python_version',
        '_job_dir',
        '_impersonation_chain',
    ]
    operator_extra_links = (AIPlatformConsoleLink(),)
    @apply_defaults
    def __init__(