GitLab

* 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>