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to widen the value is no longer valid and yields a dtype mismatch.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`.
Ruihang Lai
7cbaa21131