[Bugfix] Fix illegal memory access in FP8 MoE kernel

[comaniac]

This PR fixes the illegal memory access error that frequently happens after #6029. It seems like the following lines cause the problem:

intermediate_cache1 = intermediate_cache1[:tokens_in_chunk]
intermediate_cache2 = intermediate_cache2[:tokens_in_chunk]
intermediate_cache3 = intermediate_cache3[:tokens_in_chunk]

The chunk size is 64*1024, so in most cases we only have one chunk and this line should have no effect. So I have no idea why this causes the issue. This PR simply avoids this branch when we only have one chunk.

cc @robertgshaw2-neuralmagic @mgoin @pcmoritz

[github-actions]

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[mgoin]

Thanks for tracking this down, this is good to get FP8 MoE working again in most cases