skip additional flaky rpc tests (#28934)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28934

These tests are flaky, skip them as we investigate for a root cause
ghstack-source-id: 92945898

Test Plan: tests pass

Differential Revision: D18235766

fbshipit-source-id: 9bff65653954b767e32bcc1d25c65b0cea2c4331
diff --git a/test/dist_autograd_test.py b/test/dist_autograd_test.py
index 2f993dc..9122084 100644
--- a/test/dist_autograd_test.py
+++ b/test/dist_autograd_test.py
@@ -1010,6 +1010,7 @@
         # Verify thread is still alive (indicating backward hasn't completed yet).
         self.assertTrue(t.is_alive())
 
+    @unittest.skip("Test is flaky, see https://github.com/pytorch/pytorch/issues/28928")
     @dist_init
     def test_backward_autograd_engine_error(self):
         with dist_autograd.context() as context_id:
diff --git a/test/rpc_test.py b/test/rpc_test.py
index f8d8f71..9d191ce 100644
--- a/test/rpc_test.py
+++ b/test/rpc_test.py
@@ -484,6 +484,7 @@
         value = VALUE_FUTURE.result()
         self.assertEqual(value, expected_src_rank)
 
+    @unittest.skip("Test is flaky, see https://github.com/pytorch/pytorch/issues/28932")
     @dist_init
     def test_py_built_in(self):
         n = self.rank + 1