fix dynamo.explain examples (#122745)

Summary:
`dynamo.explain()` was updated to return a structure but the docs weren't updated to match.

- Update the docs to use the new API
- Remove some dead code left when `explain` was updated.
- Drive-by: Fix some `nopython` uses that I noticed
- Drive-by: I noticed an ignored error coming from CleanupHook on shutdown - make it check the global before setting it.

Fixes #122573

X-link: pytorch/pytorch#122745
Approved by: https://github.com/jansel

Reviewed By: atalman

Differential Revision: D55485698

Pulled By: aorenste

fbshipit-source-id: fe308dc169cfc8b88d1bb62b393e1f80eb95a9a5

userbenchmark/dynamo/dynamobench/_dynamo/utils.py

@@ -718,7 +718,9 @@ class CleanupHook:
     name: str
 
     def __call__(self, *args):
-        CleanupManager.count -= 1
+        # Make sure we're not shutting down
+        if CleanupManager is not None:
+            CleanupManager.count -= 1
         del self.scope[self.name]
 
     @staticmethod