Add Metal debug option and capture functions (#707)

* Add Metal debug option and capture functions

* Add brief Metal debugger documentation

* doc nits

---------

Co-authored-by: Awni Hannun <awni@apple.com>

examples/cpp/CMakeLists.txt

@@ -8,3 +8,4 @@ endfunction(build_example)
 build_example(tutorial.cpp)
 build_example(linear_regression.cpp)
 build_example(logistic_regression.cpp)
+build_example(metal_capture.cpp)

examples/cpp/metal_capture.cpp

@@ -0,0 +1,30 @@
+// Copyright © 2024 Apple Inc.
+
+#include <cassert>
+#include <iostream>
+
+#include "mlx/mlx.h"
+
+using namespace mlx::core;
+
+int main() {
+  // Enable the MLX_METAL_DEBUG CMake option to enhance the capture with groups,
+  // labels, etc.
+  assert(metal::start_capture());
+
+  // Start at index two because the default GPU and CPU streams have indices
+  // zero and one, respectively. This naming matches the label assigned to each
+  // stream's command queue.
+  auto s2 = new_stream(Device::gpu);
+  auto s3 = new_stream(Device::gpu);
+
+  auto a = arange(1.f, 10.f, 1.f, float32, s2);
+  auto b = arange(1.f, 10.f, 1.f, float32, s3);
+  auto x = add(a, a, s2);
+  auto y = add(b, b, s3);
+
+  // The multiply will happen on the default stream.
+  std::cout << multiply(x, y) << std::endl;
+
+  metal::stop_capture();
+}