**Description**

It would be nice if the weight_rule default value could be managed by a global config

suggested config:
```
# Weighting method used for the effective total priority weight of the task. 
# Options are: { downstream | upstream | absolute } default is
default_weight_rule = downstream
```
**Use case / motivation**

In some pipeline, you really need to have absolute weight, and then you have to add a line in each task definition which is annoying

airflow/models/baseoperator.py
"""Performs dry run for the operator - just render template fields."""
        self.log.info('Dry run')
        for field in self.template_fields:
            content = getattr(self, field)
            if content and isinstance(content, str):
                self.log.info('Rendering template for %s', field)
                self.log.info(content)
    def get_direct_relative_ids(self, upstream: bool = False) -> Set[str]:
        """
        Get set of the direct relative ids to the current task, upstream or
        downstream.
        """
        if upstream:
            return self._upstream_task_ids
        else:
            return self._downstream_task_ids
    def get_direct_relatives(self, upstream: bool = False) -> List["BaseOperator"]: