[Kernels] Use Int thread_idx etc. in examples, demos, and tests (Part 45/N)

Migrate demos and KGEN tests to use the
`Int`-returning GPU ID accessors directly instead of `_uint as` or
`_int as` import aliases. Also remove now-redundant `Int()` and
`UInt()` wrapping around these accessors.

MODULAR_ORIG_COMMIT_REV_ID: b654c338a9c15f5d0419e708a081bcf84014eff9

Author: JoeLoser

mojo/stdlib/std/gpu/primitives/id.mojo

@@ -322,11 +322,10 @@ alias:
 from std.gpu import thread_idx_uint as thread_idx
 ```
 
-To migrate to `Int`, instead import `thread_idx_int` and update uses to reflect
-the change to `Int`:
+To use the default `Int`-returning accessor, import `thread_idx` directly:
 
 ```mojo
-from std.gpu import thread_idx_int as thread_idx
+from std.gpu import thread_idx
 ```
 
 This `thread_idx` accessor will change to yielding `Int` values in a future