rename namespace f4d to velox (#61)
Summary:
Pull Request resolved: https://github.com/facebookexternal/torchdata/pull/61
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62860
Pull Request resolved: https://github.com/facebookexternal/presto_cpp/pull/453
Moving all namespace definitions, declarations and references from 'f4d' to 'velox'
Test Plan:
```
buck build //f4d/...
buck test //f4d/...
```
Also monitor the signals from sandcaslte
Reviewed By: pedroerp
Differential Revision: D30140136
fbshipit-source-id: 5b53ac768bb7e5cd07c93a9b04dfd6363080eb52
