Add support for instantiating objects with hydra

[Alicegaz]

Currently, torchok does not support instantiating objects with hydra as here https://hydra.cc/docs/advanced/instantiate_objects/overview/. Two common usages:

1. I need to provide the object to the layer. For example, generic blocks accept pooling object as parameter. And configuring those objects in the config, with hydra creating an instance of those when passing to the block, is a must.
2. I simply need to automatically instantiate a class passing the parameters from config, with no need to require user provide model_name and model_params, but just:

model:
   __target__: src.models.backbones.resnet.seresnet18
  pretrained: False