[upstream_ut] test_gather_backward_one_dim_xpu accuracy mismatch

## Description
`test_gather_backward_one_dim_xpu` fails with incorrect gradient values from the 1-D gather backward on XPU.

## Error log
```
File "test/test_torch.py", line 2023, in _test_gather_backward_one_dim
self.assertEqual(grad, expected, atol=0, rtol=0)
AssertionError: Tensor-likes are not equal!
Mismatched elements: 1741 / 2989 (58.2%)
Greatest absolute difference: 4.0 at index (423,)
Greatest relative difference: 3.5682600696418376e-07 at index (423,)
```

## Reproduce
Using pytorch/pytorch#196421 on a PVC machine:
```
python test/test_torch.py TestTorchDeviceTypeXPU.test_gather_backward_one_dim_xpu
```

## Expected
gather backward accumulates gradients correctly (bit-exact per the test's atol=0/rtol=0), matching CPU/CUDA.

_Filed with AI assistance; reproduced per the details above._

Start with test/test_torch.py and the _test_gather_backward_one_dim test around line 2023. Run TestTorchDeviceTypeXPU.test_gather_backward_one_dim_xpu on a PVC machine and compare the XPU gradient with the CPU/CUDA results and the exact assertions. Done means the XPU gather backward gradients match the expected values with atol=0 and rtol=0.

Written by the indexing model from the issue text.