test(pt_expt): restore sys.modules snapshot in plugin entry-point test

test_plugin popped deepmd.pt_expt from sys.modules without restoring it,
leaving the package's cached submodules bound to a dead parent. Any later
import of a cached submodule (e.g. deepmd.pt_expt.infer.deep_eval)
re-created a BARE parent whose utils/infer attributes were never rebound,
and mock.patch('deepmd.pt_expt.utils...') in
test_deep_eval_serialize_api failed with AttributeError under py3.10's
mock target resolution. Shard-order dependent: this PR's new test files
reshuffled CI shard 4 and exposed the latent master-side hygiene bug.
Snapshot the whole deepmd.pt_expt module tree before the re-import and
restore it (including the parent-package attribute) afterwards.

Author: Han Wang

source/tests/pt_expt/test_plugin.py

@@ -22,12 +22,36 @@ def fake_entry_points(*, group=None):
         return [_FakeEntryPoint(calls)]
 
     monkeypatch.setattr(importlib.metadata, "entry_points", fake_entry_points)
+
+    # Snapshot the deepmd.pt_expt module tree BEFORE re-importing. Just popping
+    # "deepmd.pt_expt" and leaving its submodules cached poisons sys.modules
+    # for the rest of the pytest process: a later import of a cached submodule
+    # (e.g. deepmd.pt_expt.infer.deep_eval) re-creates a BARE parent package
+    # whose submodule attributes (utils/infer/...) are never rebound, and
+    # mock.patch("deepmd.pt_expt.utils...") then fails with AttributeError on
+    # py3.10 (shard-order dependent CI failure).
+    saved = {
+        k: v
+        for k, v in sys.modules.items()
+        if k == "deepmd.pt_expt" or k.startswith("deepmd.pt_expt.")
+    }
+    deepmd_pkg = sys.modules.get("deepmd")
     sys.modules.pop("deepmd.pt_expt", None)
 
     try:
         importlib.import_module("deepmd.pt_expt")
     finally:
-        sys.modules.pop("deepmd.pt_expt", None)
+        # drop everything the fresh import created, then restore the snapshot
+        # (including the parent-package attribute binding).
+        for k in [
+            m
+            for m in list(sys.modules)
+            if m == "deepmd.pt_expt" or m.startswith("deepmd.pt_expt.")
+        ]:
+            sys.modules.pop(k, None)
+        sys.modules.update(saved)
+        if deepmd_pkg is not None and "deepmd.pt_expt" in saved:
+            deepmd_pkg.pt_expt = saved["deepmd.pt_expt"]
 
     assert groups == ["deepmd.pt_expt"]
     assert calls == ["load"]