Sampler API described for customization. (#97338)
Explanation with examples of sampler customization added.
* fixed TypeVar
* removed unused init from Sampler class
* added examples for custom sampler and batch sampler
* Distributed sampler typing fixed.
* _InfiniteConstantSampler fixed
Fixes #92268
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97338
Approved by: https://github.com/NivekT
