Cleanup the code in the `dynamo` userbenchmark (#110519)
Summary:
Skip importing the modules that are only available in the pytorch source code, not pytorch nightly release.
Make dynamo benchmark work on both OSS and internal.
X-link: https://github.com/pytorch/benchmark/pull/1960
Test Plan:
```
$ python run_benchmark.py dynamo --only alexnet --training --performance --inductor
loading model: 0it [00:05, ?it/s]
cuda train alexnet
running benchmark: 100%|█████████████████| 30/30 [00:00<00:00, 41.46it/s]
1.129x
```
```
$ buck2 run mode/opt //pytorch/benchmark:run_benchmark -- dynamo --only alexnet --training --inductor --performance --output-directory $HOME
loading model: 0it [00:16, ?it/s]
running benchmark: 100%|█████████████████| 30/30 [00:00<00:00, 37.94it/s]
cuda train alexnet
1.120x
```
Differential Revision: D49912006
Pulled By: xuzhao9
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110519
Approved by: https://github.com/desertfire, https://github.com/jansel
