Fixed wrong help message shown on '--help' (#446)

Summary:
Correct help message was obfuscated by the transient `ArgumentParser` used only for eagerly read `--user-dir` flag.

To reproduce just try:
```bash
python3 train.py --help
```
Pull Request resolved: https://github.com/pytorch/fairseq/pull/446

Differential Revision: D13674731

Pulled By: myleott

fbshipit-source-id: b9503a4d7ef26405be630d31c0ca02386d783031