Fix int32 overflow in CUDA Cast and UnaryElementWise kernels for tensors with >2^31 elements

[Copilot]

• Fix unary_elementwise_impl.cuh: Change CUDA_LONG to int64_t for N parameter and loop index in _UnaryElementWise kernel, and fix blocksPerGrid calculation
• Fix cast_op.cu: Change CUDA_LONG to int64_t for N parameter and loop index in CastKernelStd, CastKernelSat, and CudaCastPairwiseKernel kernels, and remove static_cast<int> truncation
• Use size_t for pair_count in CudaCastPairwise to avoid double conversion (review feedback)
• Rename test to CastKernelCorrectness_ModerateSize and add CastKernel_Int64IndexArithmetic_NoOverflow host-side test (review feedback)
• Merge from main to resolve conflicts with Float8E8M0 tests

[tianleiwu]

Review Summary

Correct and well-scoped fix for a real int32 overflow bug in CUDA Cast and UnaryElementWise kernels. The changes consistently replace CUDA_LONG (int32_t) with int64_t across kernel parameters and index calculations, matching the same fix pattern applied to Gather in PR #28108.

Positives:

• The static_cast<int64_t>(NumElementsPerThread) correctly anchors the multiplication chain in 64-bit arithmetic before multiplying with blockIdx.x, preventing intermediate overflow.
• The unary_elementwise_impl.cuh header change propagates the fix to all unary elementwise ops (Abs, Neg, Sqrt, Log, Exp, Erf, etc.) in a single edit.
• All three cast kernel variants (CastKernelStd, CastKernelSat, CudaCastPairwiseKernel) are consistently updated — no kernel was missed.
• Removal of static_cast<int>(num_of_elements) truncation in the launch functions is the most important part, since that's where size_t → int32_t silently lost high bits.

Broader concern (out of scope): The CALCULATE_ELEMENTWISE_INDEX_OR_EXIT macro still uses CUDA_LONG and is used by dozens of other CUDA kernels (expand, tile, scatter_nd, resize, upsample, etc.) — they have the same int32 overflow vulnerability. Consider filing a follow-up issue to track the systemic fix.

[justinchuby]

@copilot please address comments and rebase from main

[Copilot]

@copilot please address comments and rebase from main

Addressed both review comments in f28a737. The rebase from main has a merge conflict in cast_op_test.cc (due to the Float8E8M0 tests added in #28435) that requires a force-push which I'm unable to perform with the available tooling. The conflict is only in the test file — the functional CUDA kernel changes apply cleanly to main.

[justinchuby]

@copilot can you create a merge commit?

[Copilot]

@copilot can you create a merge commit?

Done in 081aa2f — merged origin/main into the branch, resolving the conflict in cast_op_test.cc (our tests now appear before the Float8E8M0 tests added in #28435).

[github-actions]

You can commit the suggested changes from lintrunner.