[BE] @serialTest decorator must be called (#157388)

Otherwise it turns test into a trivial one(that always succeeds), as following example demonstrates
```python
import torch
from torch.testing._internal.common_utils import serialTest, run_tests, TestCase

class MegaTest(TestCase):
    @serialTest
    def test_foo(self):
        if hasattr(self.test_foo, "pytestmark"):
            print("foo has attr and it is", self.test_foo.pytestmark)
        print("foo")

    @serialTest()
    def test_bar(self):
        if hasattr(self.test_bar, "pytestmark"):
            print("bar has attr and it is", self.test_bar.pytestmark)
        print("bar")

if __name__ == "__main__":
    run_tests()
```

That will print
```
test_bar (__main__.MegaTest.test_bar) ... bar has attr and it is [Mark(name='serial', args=(), kwargs={})]
bar
ok
test_foo (__main__.MegaTest.test_foo) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.013s

```

Added assert that arg is boolean in the decorator to prevent such silent skips in the future

Pull Request resolved: https://github.com/pytorch/pytorch/pull/157388
Approved by: https://github.com/clee2000

test/test_mps.py

@@ -9336,7 +9336,7 @@ class TestSDPA(TestCaseMPS):
             )
         self._compare_tensors(y.cpu(), y_ref)
 
-    @serialTest
+    @serialTest()
     def test_sdpa_fp32_no_memory_leak(self):
         def get_mps_memory_usage():
             return (torch.mps.current_allocated_memory() / (1024 * 1024),