[Feature] rewrite rope kernel; remove flashinfer dependencies (#18844)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

python/sglang/jit_kernel/include/sgl_kernel/utils.cuh

@@ -11,6 +11,7 @@
 #include <cuda_fp16.h>
 #include <cuda_fp8.h>
 #include <cuda_runtime.h>
+#include <type_traits>
 
 #ifndef USE_ROCM
 using fp32_t = float;
@@ -62,6 +63,31 @@ SGL_DEVICE void PDLTriggerSecondary() {
 #endif
 }
 
+/**
+ * \brief Load data with the specified type and offset from a void pointer.
+ * \tparam T The type to load.
+ * \param ptr The base pointer.
+ * \param offset The offset in number of elements of type T.
+ */
+template <typename T>
+SGL_DEVICE T load_as(const void* ptr, int64_t offset = 0) {
+  return static_cast<const T*>(ptr)[offset];
+}
+
+/**
+ * \brief Store data with the specified type and offset to a void pointer.
+ * \tparam T The type to store.
+ * \param ptr The base pointer.
+ * \param val The value to store.
+ * \param offset The offset in number of elements of type T.
+ * \note we use type_identity_t to force the caller to explicitly specify
+ * the template parameter `T`, which can avoid accidentally using the wrong type.
+ */
+template <typename T>
+SGL_DEVICE void store_as(void* ptr, std::type_identity_t<T> val, int64_t offset = 0) {
+  static_cast<T*>(ptr)[offset] = val;
+}
+
 namespace pointer {
 
 // we only allow void * pointer arithmetic for safety