fix key error in dpgen2 download (#297)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Improved artifact retrieval to prevent errors when accessing missing
artifacts, resulting in a more robust download process.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: zjgemi <[email protected]>

Author: zjgemi

dpgen2/utils/download_dpgen2_artifacts.py

@@ -359,9 +359,9 @@ def _dl_step_item(
     [step_key, io, name] = item.split(global_step_def_split)
     pref = _item_path(prefix, item)
     if io in ["input"]:
-        target = step.inputs.artifacts[name]
+        target = step.inputs.artifacts.get(name)
     elif io in ["output"]:
-        target = step.outputs.artifacts[name]
+        target = step.outputs.artifacts.get(name)
     else:
         raise RuntimeError("unknown io style {io}")
     try:

tests/utils/test_dl_dpgen2_arti.py

@@ -28,6 +28,9 @@
 
 
 class MockedArti:
+    def get(self, key):
+        return self.__getitem__(key)
+
     def __getitem__(
         self,
         key,

tests/utils/test_dl_dpgen2_arti_by_def.py

@@ -30,6 +30,9 @@
 
 
 class MockedArti:
+    def get(self, key):
+        return self.__getitem__(key)
+
     def __getitem__(
         self,
         key,