added Skip Save interval before automatic saving starts

[dxqbYD]

Automatic saving is used to have multiple checkpoints after training, so you can choose the best one. However, you know in advance that the first x000 steps will not be the best one. This PR adds a function to skip saving in those first x000 steps.

This example saves every 500 steps, but not within the first 3000:

Especially useful for full finetunes with large file sizes. The first few checkpoints waste both disk space and the time it takes to save.