MB text logging

Hi, I think it should be possible to log metrics for *every* MB to a file with the TextLogger and not just the last MB. In the end, it's the users decision to make a really lengthy log file, but there are use cases for this behavior. And if the option is turned of by default, it doesn't bother anyone who doesn't want this behavior.

Say I want to track how long the training on each minibatch takes and that I'm particularly interested in outliers in these timings. Then I'd be nice to have logs of every MB and it takes only a simple script to go through these files. I know it's already possible to do through WandB and Tensorboard, but it'd be nice to also make this possible with simple text files.

I already have an implementation done, which just implements `after_training_iteration` differently based on a boolean set at initialization, which defaults to False.

Let me know what you think :)