Add documentation about custom random DataPipe

[NivekT]

📚 The doc issue

Similar to #1104, DataLoader2 has built-in functionality to set random seed for DataPipes when they have set_seed method implemented.

https://github.com/pytorch/data/blob/main/torchdata/dataloader2/graph/settings.py#L22

We should provide official guidance about what we expect users to implement in cases where a custom random DataPipe is implemented/necessary.

Suggest a potential alternative/fix

No response