[Feature] Add data sampler (#30)

* Add DefaultSampler and InfiniteSampler

* Add unit test

* Add samplers to API reference

* Update docstring

* Improve according to comments

* Rename `num_replicas` to `world_size`

* Update docstring.

* Update mmengine/data/sampler.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update mmengine/data/sampler.py

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Fix typo in unit test

Co-authored-by: Wenwei Zhang <40779233+ZwwWayne@users.noreply.github.com>
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>