Fix CustomAutogradTest.ReentrantPriority rerun failures (pytorch#49581)

Summary:
Clear static variable at the end of the test to ensure test passes after re-runs

Pull Request resolved: pytorch#49581

Test Plan:
`./bin/test_api "--gtest_filter=CustomAutogradTest.ReentrantPriority" --gtest_repeat=50`
Before the change all subsequent runs of the test failed with
```
../test/cpp/api/autograd.cpp:681: Failure
Expected equality of these values:
  order.size()
    Which is: 310
  10
```

Reviewed By: mrshenli

Differential Revision: D25632374

Pulled By: malfet

fbshipit-source-id: 4814d22b5dff15e1b38a0187e51070771fd58370

Author: malfet

test/cpp/api/autograd.cpp

@@ -681,6 +681,8 @@ TEST(CustomAutogradTest, ReentrantPriority) {
   ASSERT_EQ(order.size(), 10);
   ASSERT_EQ(std::count(order.begin(), order.end(), 1), 9);
   ASSERT_EQ(order.back(), 0);
+  // Clear static variable in case test get executed in a loop
+  order.clear();
 }
 
 TEST(CustomAutogradTest, Hooks) {