Fix/dataset index: Index values were faulty when indexing the original samples instead of blocks.

[le1nux]

What does this PR do?

The index values in the pbin files had the wrong values. They did start with an offset and additionally, we added another offset of HEADER size when reading from the file buffer. See here for the initial offset during pbin index creation: https://github.com/Modalities/modalities/blob/4aa2e88efe13c3eaab4c6b425fdb82caf0d2a443/src/modalities/dataloader/create_packed_data.py#L145

and the additional offset that is used when reading from the memmap during training:

https://github.com/Modalities/modalities/blob/4aa2e88efe13c3eaab4c6b425fdb82caf0d2a443/src/modalities/dataloader/create_packed_data.py#L262

This PR fixes this issue and makes the index always start at byte 0, only applying the offset once when reading from the memmap file.

General changes

• index tuples are now always in bytes and the start of the first sample in the data section starts at byte 0 (before the was a wrong offset)
• added test for indexing the original samples
• the documentation was updated accordingly
• removed block_size from abstract classes that don't need to see the block_size concept

Breaking Changes

• None

Checklist before submitting final PR

• [x] My PR is minimal and addresses one issue / enhancement in isolation
• [x] I have merged main into this feature branch
• [x] I have reviewed my own code w.r.t. correct implementation, missing type hints, proper documentation, etc.
• [x] I have run a sample config for model training
• [x] I have fixed all failing tests (python tests/tests.py)

[le1nux]

fixes #163

[le1nux]

Yes, the inheritance structure can be improved. I suggest we do this in a separate PR together with improving the "packing" terms in those cases when there is no actual packing happening.

I added the issue https://github.com/Modalities/modalities/issues/167 for addressing this.