Added comment to docs regarding ToTensor transform (apache#12186)

* Corrections to profiling tutorial

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.

* Adding note in documentation regarding ToTensor transform.

* Removed line to fix lint error.

* Change to start CI build

docs/api/python/gluon/data.md

@@ -76,7 +76,7 @@ In the rest of this document, we list routines provided by the `gluon.data` pack
 ```
 
 Transforms can be used to augment input data during training. You
-can compose multiple transforms sequentially, for example:
+can compose multiple transforms sequentially (taking note of which functions should be applied before and after `ToTensor`).
 
 ```python
 from mxnet.gluon.data.vision import MNIST, transforms

python/mxnet/gluon/data/vision/transforms.py

@@ -245,6 +245,7 @@ def forward(self, x):
 
 class Resize(Block):
     """Resize an image to the given size.
+    Should be applied before `mxnet.gluon.data.vision.transforms.ToTensor`.
 
     Parameters
     ----------