Add some comments

src/targets/gpu/kernels/include/migraphx/kernels/index.hpp

@@ -41,10 +41,16 @@ inline __device__ __attribute__((const)) index_int compute_global_size()
 #ifdef MIGRAPHX_NGLOBAL
     return MIGRAPHX_NGLOBAL;
 #else
+    // This actualy works even when global is not divisible by local size.
+    // This doesnt actually do a multiplicatiosn. Instead it calls a device
+    // function to get the global size, which is why it works.
     return blockDim.x * gridDim.x; // NOLINT
 #endif
 }
 
+// We cant just use blockDim.x to get the local size since its broken on hip
+// when global is not divisible by local size. In this case, we calulate the
+// size for the last group. 
 inline __device__ __attribute__((const)) index_int compute_local_size()
 {
 #ifdef MIGRAPHX_NLOCAL