AssetGraph methods for mapping AssetSubsets (#19696)

## Summary & Motivation

We've got a whole bunch of mapping methods on the AssetGraph, and I've
always been annoyed that you have to use different ones depending on if
you're working with partitioned assets or not.

These methods let you do whatever mapping you want without having to
think about your specific partitions definition.

## How I Tested These Changes

python_modules/dagster/dagster/_core/definitions/asset_graph.py

@@ -22,6 +22,7 @@
 import toposort
 
 import dagster._check as check
+from dagster._core.definitions.asset_subset import ValidAssetSubset
 from dagster._core.definitions.auto_materialize_policy import AutoMaterializePolicy
 from dagster._core.errors import DagsterInvalidInvocationError
 from dagster._core.instance import DynamicPartitionsStore
@@ -305,6 +306,71 @@ def get_ancestors(
             ancestors.add(asset_key)
         return ancestors
 
+    def get_parent_asset_subset(
+        self,
+        child_asset_subset: ValidAssetSubset,
+        parent_asset_key: AssetKey,
+        dynamic_partitions_store: DynamicPartitionsStore,
+        current_time: datetime,
+    ) -> ValidAssetSubset:
+        """Given a child AssetSubset, returns the corresponding parent AssetSubset, based on the
+        relevant PartitionMapping.
+        """
+        child_asset_key = child_asset_subset.asset_key
+        child_partitions_def = self.get_partitions_def(child_asset_key)
+        parent_partitions_def = self.get_partitions_def(parent_asset_key)
+
+        if parent_partitions_def is None:
+            return ValidAssetSubset(parent_asset_key, value=child_asset_subset.size > 0)
+
+        partition_mapping = self.get_partition_mapping(child_asset_key, parent_asset_key)
+        parent_partitions_subset = (
+            partition_mapping.get_upstream_mapped_partitions_result_for_partitions(
+                child_asset_subset.subset_value if child_partitions_def is not None else None,
+                downstream_partitions_def=child_partitions_def,
+                upstream_partitions_def=parent_partitions_def,
+                dynamic_partitions_store=dynamic_partitions_store,
+                current_time=current_time,
+            )
+        ).partitions_subset
+
+        return ValidAssetSubset(parent_asset_key, value=parent_partitions_subset)
+
+    def get_child_asset_subset(
+        self,
+        parent_asset_subset: ValidAssetSubset,
+        child_asset_key: AssetKey,
+        dynamic_partitions_store: DynamicPartitionsStore,
+        current_time: datetime,
+    ) -> ValidAssetSubset:
+        """Given a parent AssetSubset, returns the corresponding child AssetSubset, based on the
+        relevant PartitionMapping.
+        """
+        parent_asset_key = parent_asset_subset.asset_key
+        parent_partitions_def = self.get_partitions_def(parent_asset_key)
+        child_partitions_def = self.get_partitions_def(child_asset_key)
+
+        if parent_partitions_def is None:
+            if parent_asset_subset.size > 0:
+                return ValidAssetSubset.all(
+                    child_asset_key, child_partitions_def, dynamic_partitions_store, current_time
+                )
+            else:
+                return ValidAssetSubset.empty(child_asset_key, child_partitions_def)
+
+        if child_partitions_def is None:
+            return ValidAssetSubset(child_asset_key, value=parent_asset_subset.size > 0)
+        else:
+            partition_mapping = self.get_partition_mapping(child_asset_key, parent_asset_key)
+            child_partitions_subset = partition_mapping.get_downstream_partitions_for_partitions(
+                parent_asset_subset.subset_value,
+                parent_partitions_def,
+                downstream_partitions_def=child_partitions_def,
+                dynamic_partitions_store=dynamic_partitions_store,
+                current_time=current_time,
+            )
+            return ValidAssetSubset(child_asset_key, value=child_partitions_subset)
+
     def get_children_partitions(
         self,
         dynamic_partitions_store: DynamicPartitionsStore,