Implement loops as Streams

[seanmor5]

This takes the regular loop run and builds it based off of this stream which will return a loop state per epoch. Basically this should be really flexible in that I can now control the running of the loop with regular Elixir Stream and Enum functions. You can see this in how much simpler the older run is now.

This also kills output_transform in the loop. I may add it to the Loop.run as an option and that case we can add some output transforms that will not break some older training code

Resolves #362

Will potentially resolve #455 depending on how my experiments go

[josevalim]

This is tricky...

If the stream is pointing to an external resource, then you are opening it and traversing it. If the stream has side-effects, this first element is effectively lost. The correct way is to do this when streaming starts. Something like this:

Stream.unfold({:init, arg1, arg2}, fn
  {:init, arg1, arg2}, data ->
    state = init_loop_state(...)
    {emit_next(state), {:state, state}
  {:state, state}, data ->
    ....
  :halt, _data ->
    ...
end)

[josevalim]

I think you would have to move everything to init, including fire_event(:started, ...).

[josevalim]

Do you need with_index if you already have state.epoch?