Rename (confusing) dag.sub_dag to dag.partial_subset (#11542)

There was a method on the DAG class called `sub_dag()` that had nothing
to do with Sub-DAGs or the SubDagOperator - It instead created a new
"partial" dag that contained only a selected subset of tasks.

To remove this confusion when seeing `dag.sub_dag()` used in code I have
renamed this function, and included a compat shim in case anyone is
using it outside of Airflow core.

airflow/cli/commands/dag_command.py

@@ -103,7 +103,7 @@ def dag_backfill(args, dag=None):
     args.start_date = args.start_date or args.end_date
 
     if args.task_regex:
-        dag = dag.sub_dag(
+        dag = dag.partial_subset(
             task_regex=args.task_regex,
             include_upstream=not args.ignore_dependencies)
 

airflow/cli/commands/task_command.py

@@ -411,7 +411,7 @@ def task_clear(args):
 
         if args.task_regex:
             for idx, dag in enumerate(dags):
-                dags[idx] = dag.sub_dag(
+                dags[idx] = dag.partial_subset(
                     task_regex=args.task_regex,
                     include_downstream=args.downstream,
                     include_upstream=args.upstream)

airflow/models/dag.py

@@ -1347,8 +1347,18 @@ class DAG(BaseDag, LoggingMixin):
             result._log = self._log
         return result
 
-    def sub_dag(self, task_regex, include_downstream=False,
-                include_upstream=True):
+    def sub_dag(self, *args, **kwargs):
+        """This method is deprecated in favor of partial_subset"""
+        warnings.warn(
+            "This method is deprecated and will be removed in a future version. Please use partial_subset",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return self.partial_subset(*args, **kwargs)
+
+    def partial_subset(
+        self, task_regex, include_downstream=False, include_upstream=True
+    ):
         """
         Returns a subset of the current dag as a deep copy of the current dag
         based on a regex that should match one or many tasks, and includes