[Example] Finish adaptive sampling (#998)

* [Example] Finish adaptive sampling

* refactor code and use argparse for command-line usage

* Update README.md

* add node_per_layer command-line argument

examples/pytorch/adaptive_sampling/README.md

+# Adaptive sampling for graph representation learning
+
+This is dgl implementation of [Adaptive Sampling Towards Fast Graph Representation Learning](https://arxiv.org/abs/1809.05343).
+
+The authors' implementation can be found [here](https://github.com/huangwb/AS-GCNN).
+
+## Performance
+
+Test accuracy on cora dataset achieves 0.84 around 250 epochs when sample size is set to 256 for each layer.
+
+## Usage
+
+`python adaptive_sampling.py --batch_size 20  --node_per_layer 40`
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