[CUDAX] Add a dummy device struct for now (NVIDIA#2066)

* Add dummy device struct
* add `__scoped_device` for changing and restoring the current device

---------

Co-authored-by: Michael Schellenberger Costa <[email protected]>
Co-authored-by: Eric Niebler <[email protected]>

cudax/include/cuda/experimental/__device/device.cuh

@@ -0,0 +1,118 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of CUDA Experimental in CUDA C++ Core Libraries,
+// under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _CUDAX__DEVICE_DEVICE
+#define _CUDAX__DEVICE_DEVICE
+
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda_runtime_api.h>
+
+#include <cuda/std/__cuda/api_wrapper.h>
+
+namespace cuda::experimental
+{
+// Dummy struct for now to be able to reference it in other places
+// TODO this might be device_ref instead
+// TODO proper implementation
+//! @brief A non-owning representation of a CUDA device
+class device
+{
+  int __id = 0;
+
+public:
+  //! @brief Create a `device` object from a native device ordinal.
+  /*implicit*/ constexpr device(int id) noexcept
+      : __id(id)
+  {}
+
+  //! @brief Retrieve the native ordinal of the device
+  //!
+  //! @return int The native device ordinal held by the device object
+  _CCCL_NODISCARD constexpr int get() const noexcept
+  {
+    return __id;
+  }
+};
+
+#ifndef DOXYGEN_SHOULD_SKIP_THIS // Do not document
+
+//! @brief RAII helper which saves the current device and switches to the
+//!        specified device on construction and switches to the saved device on
+//!        destruction.
+//!
+struct __scoped_device
+{
+private:
+  // The original device ordinal, or -1 if the device was not changed.
+  int const __old_device;
+
+  //! @brief Returns the current device ordinal.
+  //!
+  //! @throws cuda_error if the device query fails.
+  static int __current_device()
+  {
+    int device = -1;
+    _CCCL_TRY_CUDA_API(cudaGetDevice, "failed to get the current device", &device);
+    return device;
+  }
+
+  explicit __scoped_device(int new_device, int old_device) noexcept
+      : __old_device(new_device == old_device ? -1 : old_device)
+  {}
+
+public:
+  //! @brief Construct a new `__scoped_device` object and switch to the specified
+  //!        device.
+  //!
+  //! @param new_device The device to switch to
+  //!
+  //! @throws cuda_error if the device switch fails
+  explicit __scoped_device(device new_device)
+      : __scoped_device(new_device.get(), __current_device())
+  {
+    if (__old_device != -1)
+    {
+      _CCCL_TRY_CUDA_API(cudaSetDevice, "failed to set the current device", new_device.get());
+    }
+  }
+
+  __scoped_device(__scoped_device&&)                 = delete;
+  __scoped_device(__scoped_device const&)            = delete;
+  __scoped_device& operator=(__scoped_device&&)      = delete;
+  __scoped_device& operator=(__scoped_device const&) = delete;
+
+  //! @brief Destroy the `__scoped_device` object and switch back to the original
+  //!        device.
+  //!
+  //! @throws cuda_error if the device switch fails. If the destructor is called
+  //!         during stack unwinding, the program is automatically terminated.
+  ~__scoped_device() noexcept(false)
+  {
+    if (__old_device != -1)
+    {
+      _CCCL_TRY_CUDA_API(cudaSetDevice, "failed to restore the current device", __old_device);
+    }
+  }
+};
+
+#endif // DOXYGEN_SHOULD_SKIP_THIS
+
+} // namespace cuda::experimental
+
+#endif // _CUDAX__DEVICE_DEVICE