[Relax] Fix int64 row index cast in GPU multinomial sampling

[MasterJH5574]

After the tirx refactor, a T.let binding no longer implicitly casts
its right-hand side to the annotated dtype. In
gpu_multinomial_from_uniform, row_idx is annotated int64 but is
loaded from the row_indices buffer, whose dtype is the configurable
sample_indices_dtype and may be int32. Relying on the let annotation
to widen the value is no longer valid and yields a dtype mismatch.

Wrap the load in an explicit T.Cast("int64", ...) so the row index is
always converted to the annotated int64 type regardless of
sample_indices_dtype.

[gemini-code-assist]

Code Review

This pull request updates the parallel_sampling_from_prob function in python/tvm/relax/backend/gpu_generic/sampling.py to explicitly cast row_indices[bx, 0] to an int64 type using T.Cast when assigning it to row_idx. There are no review comments, and I have no feedback to provide.

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