Add support for CPU-only benchmarking.

Fixes #95.

CPU-only mode is enabled by setting the `is_cpu_only` property while
defining a benchmark, e.g. `NVBENCH_BENCH(foo).set_is_cpu_only(true)`.

An optional `nvbench::exec_tag::no_gpu` hint can also be passed to
`state.exec` to avoid instantiating GPU benchmarking backends. Note that
a CUDA compiler and CUDA runtime are always required, even if all benchmarks
in a translation unit are CPU-only.

Similarly, a new `nvbench::exec_tag::gpu` hint can be used to avoid
compiling CPU-only backends for GPU benchmarks.

examples/CMakeLists.txt

@@ -2,6 +2,7 @@ set(example_srcs
   auto_throughput.cu
   axes.cu
   custom_criterion.cu
+  cpu_only.cu
   enums.cu
   exec_tag_sync.cu
   exec_tag_timer.cu

examples/cpu_only.cu

@@ -0,0 +1,83 @@
+/*
+ *  Copyright 2025 NVIDIA Corporation
+ *
+ *  Licensed under the Apache License, Version 2.0 with the LLVM exception
+ *  (the "License"); you may not use this file except in compliance with
+ *  the License.
+ *
+ *  You may obtain a copy of the License at
+ *
+ *      http://llvm.org/foundation/relicensing/LICENSE.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#include <nvbench/nvbench.cuh>
+
+#include <chrono>
+#include <thread>
+
+// Block execution of the current CPU thread for `seconds` seconds.
+void sleep_host(double seconds)
+{
+  std::this_thread::sleep_for(
+    std::chrono::milliseconds(static_cast<nvbench::int64_t>(seconds * 1000)));
+}
+
+//=============================================================================
+// Simple CPU-only benchmark that sleeps on host for a specified duration.
+void simple(nvbench::state &state)
+{
+  const auto duration = state.get_float64("Duration");
+
+  state.exec([duration](nvbench::launch &) { sleep_host(duration); });
+}
+NVBENCH_BENCH(simple)
+  // 100 -> 500 ms in 100 ms increments.
+  .add_float64_axis("Duration", nvbench::range(.1, .5, .1))
+  // Mark as CPU-only.
+  .set_is_cpu_only(true);
+
+//=============================================================================
+// Simple CPU-only benchmark that sleeps on host for a specified duration and
+// uses a custom timed region.
+void simple_timer(nvbench::state &state)
+{
+  const auto duration = state.get_float64("Duration");
+
+  state.exec(nvbench::exec_tag::timer, [duration](nvbench::launch &, auto &timer) {
+    // Do any setup work before starting the timer here...
+    timer.start();
+
+    // The region of code to be timed:
+    sleep_host(duration);
+
+    timer.stop();
+    // Any per-run cleanup here...
+  });
+}
+NVBENCH_BENCH(simple_timer)
+  // 100 -> 500 ms in 100 ms increments.
+  .add_float64_axis("Duration", nvbench::range(.1, .5, .1))
+  // Mark as CPU-only.
+  .set_is_cpu_only(true);
+
+//=============================================================================
+// Simple CPU-only benchmark that uses the optional `nvbench::exec_tag::no_gpu`
+// hint to prevent GPU measurement code from being instantiated. Note that
+// `set_is_cpu_only(true)` is still required when using this hint.
+void simple_no_gpu(nvbench::state &state)
+{
+  const auto duration = state.get_float64("Duration");
+
+  state.exec(nvbench::exec_tag::no_gpu, [duration](nvbench::launch &) { sleep_host(duration); });
+}
+NVBENCH_BENCH(simple_no_gpu)
+  // 100 -> 500 ms in 100 ms increments.
+  .add_float64_axis("Duration", nvbench::range(.1, .5, .1))
+  // Mark as CPU-only.
+  .set_is_cpu_only(true);