[feat] add fa3 in sgl-kernel (#4902)

Co-authored-by: Sleepcoo <Sleepcoo@gmail.com>

sgl-kernel/README.md

@@ -92,6 +92,36 @@ Steps to add a new kernel:
     )
     ```
 
+### Integrating Third-Party Libraries with Data Type Conversion
+
+When integrating new third-party libraries like flash-attention, you may encounter data type compatibility issues between the C++ interface and PyTorch bindings. For example, the third-party code might use `float` or `int` types, while PyTorch requires `double` and `int64_t`.
+
+To address this issue, we provide the `make_pytorch_shim` function in [sgl_kernel_torch_shim](https://github.com/sgl-project/sglang/blob/main/sgl-kernel/include/sgl_kernel_torch_shim.h) that handles data type conversions automatically.
+
+When you need to support new data type conversions, you can easily add conversion functions like this:
+
+```cpp
+// Map `int` -> `int64_t`
+template <>
+struct pytorch_library_compatible_type<int> {
+  using type = int64_t;
+  static int convert_from_type(int64_t arg) {
+    TORCH_CHECK(arg <= std::numeric_limits<int>::max(), "int64_t value is too large to be converted  to int");
+    TORCH_CHECK(arg >= std::numeric_limits<int>::min(), "int64_t value is too small to be converted to int");
+    return arg;
+  }
+};
+```
+
+To use this with your library functions, simply wrap them with make_pytorch_shim:
+
+```cpp
+/*
+ * From flash-attention
+ */
+ m.def("fwd", make_pytorch_shim(mha_fwd));
+```
+
 ### Build & Install
 
 Development build: