Make more examples

[vfdev-5]

Suggested examples to be added to the repo:

• learn to string reverse with transformers
• learning basic arithmetics with transformers: addition, multiplication
• learning counting based on https://arxiv.org/pdf/2506.16055
• SFT

An example is done if we can:

• use trainite to generate the code of the example
• train a custom model on the built-in dataset
• train a built-in model on a custom modification of a built-in dataset