exp: refactor (local) executors to start using process manager

dvc/repo/experiments/base.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import NamedTuple, Optional
 
 from dvc.exceptions import DvcException, InvalidArgumentError
 
@@ -141,3 +141,20 @@ def from_ref(cls, ref: str):
         baseline_sha = parts[2] + parts[3] if len(parts) >= 4 else None
         name = parts[4] if len(parts) == 5 else None
         return cls(baseline_sha, name)
+
+
+class ExpStashEntry(NamedTuple):
+    """Experiment stash entry.
+
+    stash_index: EXPS_STASH index for this entry. Can be None if this commit
+        is not pushed onto the stash ref.
+    head_rev: HEAD Git commit to be checked out for this experiment.
+    baseline_rev: Experiment baseline commit.
+    branch: Optional exp (checkpoint) branch name for this experiment.
+    """
+
+    stash_index: Optional[int]
+    head_rev: str
+    baseline_rev: str
+    branch: Optional[str]
+    name: Optional[str]