13_callback.core: create docments

[warner-benjamin]

Add docments to 13_callback.core.

See the style guide and contribution guide for more details (links will be added when created).

This requires moderate to advanced familiarity with the fastai training loop and callbacks.

Docments progress:

• [x] Callback
• [x] TrainEvalCallback
• [x] Gather and fetch preds callbacks

[tyoc213]

There are few arguments to docment in code apart from self, so I think this one can be https://github.com/muellerzr/fastai-docment-sprint/labels/Beginner

[bharatr21]

I would like to work on this, wanting to learn about callbacks in general and making contributions, this is perfect 😁

[tyoc213]

Gotcha, go for it :), ping us here or in docsprint general channel

[warner-benjamin]

74 and #80 resolve this issue