Corrections to profiling tutorial (apache#11887)

Corrected a race condition with stopping profiling. Added mx.nd.waitall to ensure all operations have completed, including GPU operations that might otherwise be missing.

Also added alternative code for context selection GPU vs CPU, that had error before on machines with nvidia-smi.

docs/tutorials/python/profiler.md

@@ -94,7 +94,10 @@ Let's define a method that will run one training iteration given data and label.
 
 ```python
 # Use GPU if available
-ctx = mx.gpu() if mx.test_utils.list_gpus() else mx.cpu()
+try:
+    mx.test_utils.list_gpus(); ctx = mx.gpu()
+except:
+    ctx = mx.cpu()
 
 # Initialize the parameters with random weights
 net.collect_params().initialize(mx.init.Xavier(), ctx=ctx)
@@ -144,7 +147,8 @@ profiler.set_state('run')
 
 run_training_iteration(*next(itr))
 
-# Ask the profiler to stop recording
+# Ask the profiler to stop recording after operations have completed
+mx.nd.waitall()
 profiler.set_state('stop')
 ```