* Added graph updating method and tests. resolves#3488
* removed spaces around named args
* customizing indices for graph's idtype and ctx
* changing torch ops to generic backend ops
* changing tensors to np arrays
* created dgl merge function and tests
* Changed per-graph edge updates to single ag update
* removed update method and tests
* reformat newlines & spaces
* concatenating in one-shot instead of iteratively
Co-authored-by: Mufei Li <mufeili1996@gmail.com>
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>
nv-dlasalle