[diffusion] kernel: timestep embedding kernel implementation (#12995)

Co-authored-by: 戚余航 <qiyuhang@bytedance.com>
Co-authored-by: Qi Yuhang <45795032+HydraQYH@users.noreply.github.com>

sgl-kernel/python/sgl_kernel/__init__.py

@@ -32,6 +32,7 @@ from sgl_kernel.elementwise import (
     rmsnorm,
     rotary_embedding,
     silu_and_mul,
+    timestep_embedding,
 )
 from sgl_kernel.expert_specialization import (
     es_fp8_blockwise_scaled_grouped_mm,