Add `SingleProcessReadingService` for `DataLoader2` by default

[ejguan]

🚀 The feature

Add SingleProcessReadingService to adapt the graph for the sake of:

• Shuffle seed setting per epoch
• Set shuffle/sharding
• etc.

Pros: This would prevent making DataLoader2 more complicated. The reading service should take charge of all adaptation of the graph of DataPipes.

Cons: By adding SingleProcessReadingService, we need mixed ReadingServices for the most of cases.

Motivation, pitch

To align the behavior of DataLoader2 with or without MPRS.

Alternatives

No response

Additional context

No response