Incremental model unfreeze

[VladVin]

Is your feature request related to a problem? Please describe.
A feature that let's you set which modules of the model to unfreeze on which epoch

Describe the solution you'd like
It might be implemented as something that accepts a dict config of the following structure and unfreezes modules in the given keys when the related epoch starts:

unfreeze_schedule:
  5:
    - 'neck.module1'
    - 'neck.module2'
  15:
    - 'backbone'

Describe alternatives you've considered

1. Unfreeze only high level parts of the model, e.g. backbone, neck, head, etc.

Additional context