feat: torch-free dataloader utilities for CPU-only tooling

[rrutmann]

What does this PR do?

This PR guards the torch import in logger_utils so IndexGenerator and LargeFileLinesReader can be imported without torch, which modalities declares only in its cpu/cu12x extras. CPU-only preprocessing tooling no longer installs torch to call them.

Also adds TokenizerInstantiationModel so a tool can reuse a packing config for just its tokenizer.

General Changes

• logger_utils.get_logger now imports torch defensively; when torch isn't installed, the rank prefix is simply omitted instead of raising ImportError. This unblocks importing create_index.IndexGenerator and large_file_lines_reader.LargeFileLinesReader — both used by CPU-only data-preprocessing tooling — without needing a torch install at all. Behavior is unchanged when torch is present.
• TokenizerInstantiationModel (config/instantiation_models.py): lets a tool build just the tokenizer out of an existing packing config, without needing to satisfy the rest of that config's settings (e.g. a source file path it has no interest in).

Breaking Changes

• None

Checklist before submitting final PR

• My PR is minimal and addresses one issue in isolation
• I have merged the latest version of the target branch into this feature branch
• I have reviewed my own code w.r.t. correct implementation, missing type hints, proper documentation, etc.
• I have run a sample config for model training
• I have checked that all tests run through (python tests/tests.py)
• I have updated the internal changelog (CHANGELOG_DEV.md)