Fix CUCTC top-k shared memory offset

[lanarkite99]

Fixes #4181

This fixes the shared-memory layout used by first_matrix__bitonic_topk_kernel.

block_topk_key is a float*, so pointer arithmetic is in units of float, not bytes. The previous code computed block_topk_key + sizeof(float) * beam, which advanced the value buffer by sizeof(float) * beam float elements instead of by beam float elements.

The launch allocates the result region as:

beam * sizeof(float) + beam * sizeof(int)

[pytorch-bot]

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