improve doc on how to understand dynamo (#106860)
Per the discussion in https://github.com/pytorch/pytorch/pull/106673#issuecomment-1669939815 , I add more documentation to explain the output of dynamo compilation. I didn't find any de-compile library, so I manually de-compile the bytecode. The result looks good.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106860
Approved by: https://github.com/jansel, https://github.com/msaroufim
