Enable Append Mode in SaverIterDataPipe

[rravu3]

🚀 The feature

Currently, Saver only allows write mode and only users to choose byte vs text mode. It might be useful to allow the flexibility to append to an existing file.

Motivation, pitch

Allowing the flexibility to append could be useful for building data pipes that can enable large scale transformations on original dataset like tokenizing, randomizing and/or splitting across training and evaluation.

Alternatives

No response

Additional context

No response