**Apache Airflow version**:
v2.0.0

**What happened**:

Custom operators inheriting from DummyOperator are go in to a scheduled state, where as DummyOperator itself is set straight to success.

**What you expected to happen**:

All operators should be successfully executed.

**How to reproduce it**:

```py
from datetime import datetime
from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator

class MyJob(DummyOperator):
    template_fields_renderers = {
        "body": "json"
    }
    template_fields = ("body",)

    def __init__(self, body, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.body = body

with DAG(
    dag_id="test",
    schedule_interval=None,
    start_date=datetime(2020, 8, 13),
) as dag:
    MyJob(task_id="aaaa", body={"aaa": "bbb"})

```

**Anything else we need to know**:
This is a known problem that we will fix, see: https://github.com/apache/airflow/pull/10956#discussion_r502492775
airflow/models/baseoperator.py
"""
        Lock task for execution to disable custom action in __setattr__ and
        returns a copy of the task
        """
        other = copy.copy(self)
        other._lock_for_execution = True  
        return other
    def set_xcomargs_dependencies(self) -> None:
        """
        Resolves upstream dependencies of a task. In this way passing an ``XComArg``
        as value for a template field will result in creating upstream relation between
        two tasks.
        **Example**: ::
            with DAG(...):
                generate_content = GenerateContentOperator(task_id="generate_content")
                send_email = EmailOperator(..., html_content=generate_content.output)
            # This is equivalent to
            with DAG(...):
                generate_content = GenerateContentOperator(task_id="generate_content")
                send_email = EmailOperator(
                    ..., html_content="{{ task_instance.xcom_pull('generate_content') }}"
                )
                generate_content >> send_email
        """
        from airflow.models.xcom_arg import XComArg
        def apply_set_upstream(arg: Any):