Ability to train based on epoch

[abatilo]

At the moment, you can train using a number of steps, but it would be great if we could train on a number of epochs of the passed in dataset.

Additionally, some way to know how far into a dataset you are when training would be amazing.

[jaysonfrancis]

I use something similar to this for approximating;

epoch_approx = step / (total_tokens // global_batch_size)

where global_batch_size = local_batch * seq_len * world_size