Save optimizer state, improve docs and typing

New Features

• Add saving of optimizer state for argus.Model and checkpoint callbacks.

  model.save('models/model.pth', optimizer_state=True)
  
  checkpoint = Checkpoint(dir_path='models/', optimizer_state=True)

• Add get_device method to argus.Model.
• Typing and fixing most cases of mypy errors.

Fix

• Remove torch.optim._multi_tensor optimizers from defaults (torch >= 1.7.0).

Docs

• Section argus.engine.
• Section argus.metrics.
• Section argus.utils with deep conversions.
• Add docs for decorator callbacks.
• Add docs for argus.Model methods: __init__, set_device, get_device, get_nn_module.
• Update examples section.
• Proofread and improve docs. Many small docstring fixes.

Internal changes

• Use abstract container classes from collections.abc.
• Now Engine and State only work with the argus.Model methods as a step_method. Phase name takes from the method name.
• Simplify default logging.

Breaking Changes

• Change optimizer state in argus.load_model. Now change_state_dict_func takes two arguments nn_state_dict and optimizer_state_dict (example).
• Remove handler_kwargs_dict from the attach method of argus.callbacks.Callback.