[FEA] New Graph Interface and Loaders for Distributed Sampling in DGL

[alexbarghi-nv]

• [x] New Graph class that duck-types DGLGraph
• [x] New WholeGraph feature store that implements DGL's FeatureStorage
• [x] New Sampler interface
• [x] New Loader interface
• [ ] New Sampler for distributed uniform neighbor sampling without dask
• [ ] New Loader for distributed uniform neighbor sampling without dask