comment.

Author: trivialfis

src/tree/gpu_hist/gradient_based_sampler.cu

@@ -318,9 +318,10 @@ GradientBasedSample ExternalMemoryGradientBasedSampling::Sample(Context const* c
   return {sample_rows, page_.get(), dh::ToSpan(gpair_)};
 }
 
-GradientBasedSampler::GradientBasedSampler(Context const* ctx, bool is_external_memory,
+GradientBasedSampler::GradientBasedSampler(Context const* /*ctx*/, bool is_external_memory,
                                            size_t n_rows, const BatchParam& batch_param,
                                            float subsample, int sampling_method) {
+  // The ctx is kept here for future development of stream-based operations.
   monitor_.Init("gradient_based_sampler");
 
   bool is_sampling = subsample < 1.0;