* [BugFix] Cast to the destination dtype in the scalar T.copy path
The scalar fast path in T.copy lowers a single-element copy directly to a
BufferStore. Unlike the general region path, which casts the loaded value to
the destination dtype in src/op/copy.cc, it stored the source value unchanged.
A cross-dtype scalar copy therefore failed the strict BufferStore dtype check.
This affects the common gather pattern of copying an int64 index tensor
(torch's default for argsort and topk) into an int32 on-device scratch buffer.
Mirror the general path by casting the source value to the destination
BufferLoad dtype when they differ, preserving any index lanes.
Add regression coverage for int64-to-int32 narrowing and int16-to-int32
widening.
Fixes#2689
* [BugFix] Convert the test index dtype with as_torch()
The TileLang dtype subclasses str, so getattr(torch, index_dtype) works, but
as_torch() states the conversion directly instead of relying on that detail.
Chennes
92072ab271