Display image previews during training.

[timntorres]

Assumes #3516 is indeed a bug and fixes it. Specifically, introduces a case that always updates image (line 329) when training.

I considered a one line fix, but three conditions on one line (if opts.show_progress_every_n_steps > 0 and shared.state.sampling_step - shared.state.current_image_sampling_step >= opts.show_progress_every_n_steps and shared.state.current_latent is not None:) seemed clunkier and less clear than separating out the edge case from the rest.

Tested and works... ...training with "Show image creation progress every N sampling steps." enabled and disabled ...training with "Show progressbar" enabled and disabled ...training with "Read parameters (prompt, etc...) from txt2img tab when making previews" enabled and disabled ...using regular txt2img