[auto-materialize] fix external asset + amp (#20204)

## Summary & Motivation

## How I Tested These Changes

python_modules/dagster/dagster/_core/definitions/data_time.py

@@ -539,7 +539,7 @@ def get_minutes_overdue(
                 "Cannot calculate minutes late for asset without a FreshnessPolicy"
             )
 
-        if self.asset_graph.is_external(asset_key):
+        if self.asset_graph.is_observable(asset_key):
             current_data_time = self._get_source_data_time(asset_key, current_time=evaluation_time)
         else:
             current_data_time = self.get_current_data_time(asset_key, current_time=evaluation_time)

python_modules/dagster/dagster/_utils/caching_instance_queryer.py

@@ -185,7 +185,7 @@ def get_asset_record(self, asset_key: AssetKey) -> Optional["AssetRecord"]:
         return self._asset_record_cache[asset_key]
 
     def _event_type_for_key(self, asset_key: AssetKey) -> DagsterEventType:
-        if self.asset_graph.is_external(asset_key):
+        if self.asset_graph.is_observable(asset_key):
             return DagsterEventType.ASSET_OBSERVATION
         else:
             return DagsterEventType.ASSET_MATERIALIZATION
@@ -535,7 +535,7 @@ def asset_partitions_with_newly_updated_parents_and_new_cursor(
         """Finds asset partitions of the given child whose parents have been materialized since
         latest_storage_id.
         """
-        if self.asset_graph.is_external(child_asset_key):
+        if not self.asset_graph.get_parents(child_asset_key):
             return set(), latest_storage_id
 
         child_partitions_def = self.asset_graph.get_partitions_def(child_asset_key)
@@ -805,7 +805,7 @@ def get_asset_partitions_updated_after_cursor(
         if not updated_after_cursor:
             return set()
         if after_cursor is None or (
-            not self.asset_graph.is_external(asset_key)
+            not self.asset_graph.is_observable(asset_key)
             and not respect_materialization_data_versions
         ):
             return updated_after_cursor