[Kernel]Remove recast iter from sliding window attention kernel.

[amd-nprotaso]

#933 was merged, now we can use add_offset without recast_iter, with equivalent performance:

B	seq	win	prev ms	cur ms	prev/cur	prev TF	cur TF
1	1024	(512,0)	0.0297	0.0300	0.99x	217.6	215.3
1	2048	(512,0)	0.0353	0.0350	1.01x	427.3	430.6
1	2048	(1024,0)	0.0469	0.0475	0.99x	549.9	543.0
1	4096	(512,0)	0.0734	0.0697	1.05x	440.0	463.3
1	4096	(1024,0)	0.1049	0.1012	1.04x	573.9	594.5
1	8192	(512,0)	0.1581	0.1616	0.98x	422.0	412.7
1	8192	(1024,0)	0.2325	0.2398	0.97x	554.8	538.0
1	8192	(4096,0)	0.5402	0.5459	0.99x	763.5	755.5
1	16384	(4096,0)	1.1505	1.1471	1.00x	836.4	838.9
1	32768	(8192,0)	4.3768	4.3778	1.00x	879.4	879.2
1	65536	(8192,0)	9.2751	9.2596	1.00x	889.2	890.7
1	131072	(8192,0)	19.0403	19.0032	1.00x	895.2	896.9

[Phil-amd]

LGTM. v_smem is already a bf16 pointer, and add_offset preserves its element type, so removing the redundant recast_iter is semantically safe. The gfx950 tests and performance results also look good.