fix: client.logout() re-enables client.login() (#10173)

client.login() controls a singleton, and throws an error if you try to
login twice.

But if you wanted to logout() and log back in again, that seems like it
should be allowed.

An OSS user hit this issue with a script that was meant to run for
weeks, where multiple logout() and login() calls avoided dealing with
the TTL of our sessions.

harness/determined/experimental/client.py

@@ -308,15 +308,17 @@ def whoami() -> User:
 # DOES NOT REQUIRE SINGLETON (don't force a login in order to log out).
 def logout() -> None:
     """Log out of the current session."""
-    if _determined is not None:
-        return _determined.logout()
-
-    logger.warning(
-        "client has not been logged in, either explicitly by client.login() or implicitly by any "
-        "other client.* function, so client.logout() has no session to log out of and is a no-op. "
-        "If you would like to log out of the default active session, try "
-        "client.Determined().logout() instead."
-    )
+    global _determined
+    if _determined is None:
+        logger.warning(
+            "client has not been logged in, either explicitly by client.login() or implicitly by "
+            "any other client.* function, so client.logout() has no session to log out of and is a "
+            "no-op. If you would like to log out of the default active session, try "
+            "client.Determined().logout() instead."
+        )
+    else:
+        _determined.logout()
+        _determined = None
 
 
 @_require_singleton