[CK] use int64 for ptr offset
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## Motivation
When the number of experts (E) is large (e.g., E=257 in DeepSeek-V3),
the `expert_id * expert_stride` calculation in MOE GEMM kernels
overflows `int32` (`index_t`), causing the weight matrix (B) pointer to
wrap to an invalid address and triggering a GPU memory access fault.
For example, with `N=1024, K=7168, IsInputGemm=true`:
- `expert_stride = N * K * 2 = 14,680,064`
- `INT32_MAX / expert_stride ≈ 146`
- Any `expert_id >= 147` causes overflow → negative offset → illegal
memory access → GPU crash
## Technical Details
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## Test Plan
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## Test Result
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## Submission Checklist
- [x] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
Co-authored-by: Thomas Ning <Thomas.Ning@amd.com>
Co-authored-by: amd-shiraz <shiraz.ali@amd.com>
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