[RFC] Default to infinite epochs, not 1000

Nobody has claimed this yet.

Currently max_epochs defaults to 1000:

> If both max_epochs and max_steps aren't specified, max_epochs will default to 1000. To enable infinite training, set max_epochs = -1.

As a user, though, I would expect that if I don't specify a specific ending point, the training would continue indefinitely. In my own experiments, when the training cut off at 999 epochs, I was confused, and googling the issue didn't readily turn up this line in the documentation. When I checked my logs of all the hyperparams, max_epochs was set to None (I guess this override is applied internally). So as a user I feel like this is bad UX - I can't see a reason to put an arbitrary cutoff versus defaulting to infinite training.

It's especially frustrating when you've invested significant time into a training run, only to have it prematurely cut off due to this unexpected max_epochs limit.

cc @borda

The issue does not name a file, test, or entry point. Start by locating the configuration and documentation for max_epochs and max_steps, then review how their defaults interact. Done means agreeing on and implementing the default behavior for unspecified training limits, with documentation and tests covering the result.

Written by the indexing model from the issue text.