[BugFix] [Kernel] Fix GPU SEGV occuring in fused_moe kernel

[rasmith]

When running a large model (~500G), I encountered GPU SEGV. It was occurring in fused_moe_kernel:

Thread 98 "python" received signal SIGSEGV, Segmentation fault.
Warning: precise memory violation signal reporting is not enabled, reported
location may not be accurate.  See "show amdgpu precise-memory".
[Switching to thread 98, lane 0 (AMDGPU Lane 7:2:1:1/0 (0,0,0)[0,0,0])]
fused_moe_kernel () at fused_moe.py:142
142             b = tl.load(b_ptrs,

It seemed to me that this could be integer overflow. Changing offsets to use tl.int64 in fused_moe.py:fused_moe_kernel fixes the issue.

NOTE: There was a previous PR, but it had DCO issues and I needed to recreate it.

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