Add flag for dynamo+ddp optimizations
Add a flag that can be used to turn dynamo+ddp optimizations on. This
will be used to compare how dynamo+ddp performs with and without the
additional graph break strategy for improving dynamo+ddp
compute/communication overlap.
ghstack-source-id: d614859f78672bac58335430beb585b6393d067c
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1221
