Assume `LitModular` by default in configuration

[dxoigmn]

Right now our configuration is rather verbose when it comes to models:

model:
  _target_: mart.models.LitModular
  _convert_: all
  output_preds_key: preds
  output_target_key: target
  modules:
    ...
  training_sequence:
    ...
  ...

We could clean this up by assuming LitModular by default and moving all keys under model up 1 level like:

output_preds_key: preds
output_target_key: target
modules:
  ...
training_sequence:
  ...
...