[CK] fix moe memset size which is bigger than alloc
## Motivation
Fix an out-of-bounds hipMemsetAsync in DeviceMoeGemmBlockScale that
crashes split-K MOE GEMM with "HIP runtime error: invalid argument".
When KBatch > 1, the invoker zeroes the output buffer using arg.M *
arg.N as the byte count. However, arg.M is the padded sorted-token-id
length from MOE routing, which can be much larger than the actual output
allocation (NumTokens * TopK * N). This causes hipMemsetAsync to write
beyond the buffer, and the silently-swallowed HIP error propagates to
the subsequent kernel launch via hipGetLastError().
This patch replaces arg.M with arg.NumTokens * arg.TopK so the memset
matches the actual output size.
## Technical Details
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## Test Plan
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## Test Result
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## Submission Checklist
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https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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