* 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>

* add unit tests of data abstract interface

* update

* update

* update docs of data element

* a draft of UT of datasample, to be finished

* update datasample test

* updata

* update

* fix comments

* fix comments

* fix comments

Co-authored-by: liukuikun <liukuikun@sensetime.com>

* move fileio directory from utils

* remove CephBackend from fileio and clear some deprecated docstring

* fix format

* fix a spellling error

* tmp commit

* add unittest for basedataset

* fix lint

* resolve all comments

* update

* update

* update

* update

* update

* update

* update

* resolve comments

* resolve comments

* update

* resolve comments

* resolve comments