Add linkprediction for ogb

ogb_examples/linkproppred/ogbl-ppa/README.md

+# Graph Link Prediction for Open Graph Benchmark (OGB) PPA dataset
+
+[The Open Graph Benchmark (OGB)](https://ogb.stanford.edu/) is a collection of benchmark datasets, data loaders, and evaluators for graph machine learning. Here we complete the Graph Link Prediction task based on PGL.
+
+
+### Requirements
+
+paddlpaddle >= 1.7.1
+pgl 1.0.2
+ogb
+
+### How to Run
+
+CUDA_VISIBLE_DEVICES=0,1,2,3 python train.py --use_cuda 1  --num_workers 4 --output_path ./output/model_1 --batch_size 65536 --epoch 1000 --learning_rate 0.005  --hidden_size 256