Add a _scope_ keyword for instantiation to specify a root module

[ryan-minato]

Perhaps a scope keyword could be added to specify a common root module for a dictionary and its sub-dictionaries, similar to what is done in mmengine.

One quick (but maybe ugly) implementation I can think of is to search for all _scope_ when reading the configuration and then prepend it to the target of the same level and sub-level.

e.g.

the following two configurations would be equivalent:

conf1:

model:
_target_: pkg.model.llama.LlamaModel
attn_module:
    _target_: pkg.model.llama.MyAttention
    hidden_size: 32
position_embeddings:
    _target_: pkg.model.llama.MyRotaryEmbedding
    max_position_embeddings: 2048
num_hidden_layers: 32
...

conf2:

model:
_scope_: pkg.model.llama
_target_: LlamaModel
attn_module:
    _target_: MyAttention
    hidden_size: 32
position_embeddings:
    _target_: MyRotaryEmbedding
    max_position_embeddings: 2048
num_hidden_layers: 32
...