[Do not merge][Feature] Sort Edges by SrcDst And Enable CSR/CSC without Data Arrays

[mufeili]

Description

This PR adds a utility function for generating a memory-optimized graph in CSC format without EID array. By default the function reorders the edges and the associated features with "srcdst" order.

1. Add an API DGLGraph.sort_by_srcdst to return a graph sorted with srcdst order.
2. Allow disabling data arrays for CSR/CSC and add corresponding unit tests
3. Disable EID arrays in creating CSR/CSC formats for srcdst-sorted graphs

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To the my best knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change
• Related issue is referred in this PR

[jermainewang]

Checkout the implementation here:

dgl/python/dgl/nn/pytorch/conv/relgraphconv.py

Lines 344 to 345 in 1502c56

	sorted_etypes, index = th.sort(etypes)
	g = edge_subgraph(g, index, preserve_nodes=True)

I feel we could implement it inside edge_subgraph.

[jermainewang]

Since there are still operators cannot support empty eid array, we will push this PR after 0.6.