use shorter title (#58)

nnvm/tutorials/from_mxnet.py

 """
-Compiling MXNet Models with NNVM
-================================
+Compile MXNet Models
+====================
 **Author**: `Joshua Z. Zhang <https://zhreshold.github.io/>`_
 
 This article is an introductory tutorial to deploy mxnet models with NNVM.

nnvm/tutorials/from_onnx.py

 """
-Compiling ONNX Models with NNVM
-================================
+Compile ONNX Models
+===================
 **Author**: `Joshua Z. Zhang <https://zhreshold.github.io/>`_
 
 This article is an introductory tutorial to deploy ONNX models with NNVM.

nnvm/tutorials/imagenet_inference_gpu.py

 """
-Compile ImageNet Inference on GPU
-==================================
+Compile GPU Inference
+=====================
 **Author**: `Yuwei Hu <https://huyuwei.github.io/>`_
 
 This is an example of using NNVM to compile MobileNet/ResNet model and deploy its inference on GPU.