[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/dynamo/test_repros.py

@@ -3939,7 +3939,7 @@ class ReproTests(torch._dynamo.test_case.TestCase):
         opt_model(17, (12,), out2)
 
     @requires_cuda
-    @serialTest
+    @serialTest()
     def test_mem_leak_guards(self):
         def gn(x0, x):
             return x0 * x

test/test_cuda.py

@@ -174,7 +174,7 @@ class TestCuda(TestCase):
         for thread in threads:
             thread.join()
 
-    @serialTest
+    @serialTest()
     def test_host_memory_stats(self):
         # Helper functions
         def empty_stats():
@@ -4292,7 +4292,7 @@ class TestCudaMallocAsync(TestCase):
         finally:
             torch.cuda.memory._record_memory_history(None)
 
-    @serialTest
+    @serialTest()
     def test_max_split_expandable(self):
         try:
             torch.cuda.memory.empty_cache()
@@ -4328,7 +4328,7 @@ class TestCudaMallocAsync(TestCase):
         finally:
             torch.cuda.memory.set_per_process_memory_fraction(orig)
 
-    @serialTest
+    @serialTest()
     def test_garbage_collect_expandable(self):
         try:
             torch.cuda.memory.empty_cache()

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),

torch/testing/_internal/common_utils.py

@@ -1696,6 +1696,10 @@ def serialTest(condition=True):
     """
     Decorator for running tests serially.  Requires pytest
     """
+    # If one apply decorator directly condition will be callable
+    # And test will essentially be essentially skipped, which is undesirable
+    assert type(condition) is bool
+
     def decorator(fn):
         if has_pytest and condition:
             return pytest.mark.serial(fn)