Switch cudagraph backend to cudagraph trees (#121019)
Switch torch.compile(..., backend="cudagraphs") to use cudagraph trees. Enabled a few test in cudagraph_trees and note that there is another test suite existing for cudagraphs backend: https://github.com/pytorch/pytorch/blob/main/test/dynamo/test_cudagraphs.py.
This is basically the inductor cudagraphs without inductor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121019
Approved by: https://github.com/ezyang, https://github.com/jansel
ghstack dependencies: #121017, #121018
