* dvc: add Repo._skip_graph_checks flag
Will skip modified graph checks on add, run and import. The aim is to
avoid collecting stages and building graph on these commands.
The skip_checks flag is aimed to be private, but let people use dvc on
big repos without monkey patching or writing yaml files themselves.
* dvc: explain skip_graphs_checks flag in a comment
* test: that skip_graph_checks works
* repo: improve hack description and test
Co-authored-by: Ruslan Kuprieiev <[email protected]>