[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/include/sgl_kernel_ops.h

@@ -1006,3 +1006,15 @@ std::vector<at::Tensor> fwd_kvcache_mla_fp8(
 
 std::vector<at::Tensor> get_mla_decoding_metadata_dense_fp8(
     at::Tensor& seqlens_k, const int64_t num_heads_per_head_k, const int64_t num_heads_k);
+
+/*
+ * From csrc/sgl_diffusion/elementwise
+ */
+torch::Tensor timestep_embedding(
+    const torch::Tensor& t,
+    torch::Tensor& output,
+    int64_t dim,
+    bool flip_sin_to_cos,
+    double downscale_freq_shift,
+    double scale,
+    int64_t max_period);