Make DDPOptimizer work with torch._dynamo.explain() (#94749)
GraphModules that were created during DDPOptimizer graph breaking
lacked `compile_subgraph_reason`, which caused an exception when
running .explain().
Now the reason is provided and users can use .explain() to find out
that DDPOptimizer is causing graph breaks.
Fixes #94579
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94749
Approved by: https://github.com/voznesenskym
