[GraphBolt] mag240M example does not reach expected accuracies.

## 🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

## Description

Our graphbolt examples need to be able to match the accuracy of the mag240M reference implementation below. However, none of the [DGL](https://github.com/dmlc/dgl/tree/master/examples/graphbolt/rgcn) and the [PyG](https://github.com/dmlc/dgl/blob/master/examples/graphbolt/pyg/hetero/node_classification.py) hetero examples can match the 0.7 accuracy achieved in the reference implementation.

https://github.com/dmlc/dgl/tree/master/examples/pytorch/ogb_lsc/MAG240M