Suppress PytestCollectionWarning for non-test classes (pytorch#18192)

Fixes pytorch#18175

Added `__test__ = False` to non-test classes that pytest was incorrectly
trying to collect as test classes, causing pages of
PytestCollectionWarning output.

Classes fixed:
- `TestResult`, `TestCaseSummary`, `TestSessionState` in
`backends/test/suite/reporting.py`
- `Tester` in `backends/xnnpack/test/tester/tester.py`
- `TestCat` in `exir/tests/test_prune_empty_tensors_pass.py`
- `TestModel1` in `exir/tests/test_remove_view_copy.py`

### Test plan
Ran `pytest --collect-only` before and after the changes. Before:
multiple PytestCollectionWarning lines for each of these classes. After:
0 warnings found.

To verify:
```bash
pytest --collect-only 2>&1 | python -c "import sys; lines=[l for l in sys.stdin if 'PytestCollectionWarning' in l]; print(f'{len(lines)} warnings found')"

Author: MahinAshraful

backends/test/suite/reporting.py

@@ -60,6 +60,8 @@
 class TestResult(IntEnum):
     """Represents the result of a test case run, indicating success or a specific failure reason."""
 
+    __test__ = False
+
     SUCCESS = 0
     """ The test succeeded with the backend delegate part or all of the graph. """
 
@@ -155,6 +157,8 @@ class TestCaseSummary:
     Contains summary results for the execution of a single test case.
     """
 
+    __test__ = False
+
     backend: str
     """ The name of the target backend. """
 
@@ -210,6 +214,8 @@ def is_delegated(self):
 
 @dataclass
 class TestSessionState:
+    __test__ = False
+
     seed: int
 
     # True if the CSV header has been written to report__path.

backends/xnnpack/test/tester/tester.py

@@ -102,6 +102,8 @@ class ToExecutorch(BaseStages.ToExecutorch):
 
 
 class Tester(TesterBase):
+    __test__ = False
+
     def __init__(
         self,
         module: torch.nn.Module,

exir/tests/test_prune_empty_tensors_pass.py

@@ -15,6 +15,8 @@
 
 
 class TestCat(nn.Module):
+    __test__ = False
+
     def forward(self, x, y, z):
         empty = torch.empty((0, 6))
         return torch.cat([empty, x, empty, y, z, empty])

exir/tests/test_remove_view_copy.py

@@ -15,6 +15,8 @@
 
 
 class TestModel1(nn.Module):
+    __test__ = False
+
     def __init__(self):
         super().__init__()
         self.parameter = nn.Parameter(torch.rand(5, 6))