Move utils/seed.py to training/seed.py

[RdoubleA]

We are splitting up the massive utils directory into a separate training directory for helpers used in recipes and utils for all other miscellaneous utilities. The motivation for this is extensively discussed in #1421. We can do this move piece-wise to make it easier to do and review. I have done a few of them that you can use as a template to mimic: #1435, #1434

This task is to simply move the utils/seed.py file to its new location, training/seed.py and updating all callsites, docs, and configs that reference this.

Here's how I would make the changes:

1. Move the actual file from utils to training
2. Move the relevant APIs from utils/__init__.py to training/__init__.py
3. Move the corresponding tests from tests/torchtune/utils/ to tests/torchtune/training/
4. Move the relevant APIs from docs/source/api_ref_utilities.rst to docs/source/api_ref_training.rst
5. Now you can cmd+F find all the locations for each API and make sure the imports are updated to be from training instead of utils. Most of these callsites are in the recipes as utils.function, so you can get a lot of them by doing replace utils.function -> training.function. But there's usually a few more across the codebase