Track remaining large-index paths in nearest upsample kernels #6164

Follow-up to #5822. This issue tracks remaining large-index risks that are not addressed by the re-land.

• Audit 3d nearest upsample index_t dispatch/selection and batch-spanning indices in forward and backward; add focused coverage for selection boundaries.
• Audit the contiguous 2d backward kernel, which currently uses int for flattened indexing and batch strides; use sufficiently wide indices or validate safe limits, with regression coverage.

Relevant code: src/ATen/native/xpu/sycl/UpSampleNearest3dKernels.cpp and src/ATen/native/xpu/sycl/UpSampleNearest2dKernels.cpp. The existing #5822 tests do not establish that these remaining paths are safe.