[macOS GPU Support] Fix `MTLCommandBuffer` bottlenecks for Apple silicon GPUs (#3960)

* Flushing optimization

* Remove unnecessary checks

platforms/opencl/src/OpenCLNonbondedUtilities.cpp

@@ -355,6 +355,12 @@ void OpenCLNonbondedUtilities::prepareInteractions(int forceGroups) {
     forceRebuildNeighborList = false;
     lastCutoff = kernels.cutoffDistance;
     context.getQueue().enqueueReadBuffer(interactionCount.getDeviceBuffer(), CL_FALSE, 0, sizeof(int), pinnedCountMemory, NULL, &downloadCountEvent);
+
+    #if __APPLE__ && defined(__aarch64__)
+    // Segment the command stream to avoid stalls later.
+    if (groupKernels[forceGroups].hasForces)
+        context.getQueue().flush();
+    #endif
 }
 
 void OpenCLNonbondedUtilities::computeInteractions(int forceGroups, bool includeForces, bool includeEnergy) {
@@ -370,6 +376,11 @@ void OpenCLNonbondedUtilities::computeInteractions(int forceGroups, bool include
         context.executeKernel(kernel, numForceThreadBlocks*forceThreadBlockSize, forceThreadBlockSize);
     }
     if (useCutoff && numTiles > 0) {
+        #if __APPLE__ && defined(__aarch64__)
+        // Ensure cached up work executes while you're waiting.
+        if (kernels.hasForces)
+            context.getQueue().flush();
+        #endif
         downloadCountEvent.wait();
         updateNeighborListSize();
     }