Fix splitter_base and add unit test for trt splitter (#67569)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67569
Splitter_base has assumption that the first subgraph after split must be cpu subgraph if there exists cpu node. This is wrong, start subgraph should be determined by which subgraph has 0-dep node.
Also add unit test for splitter.
Reviewed By: yinghai
Differential Revision: D32012549
fbshipit-source-id: e2639ccd7774b4295ca05c2ddbefff9726702b3f
