Custom names for AdapterFusion layers

[calpt]

Resolves #316.

This PR implements the option to specify a custom name for an added AdapterFusion layer.

The name can be specified when adding a fusion layer like this:

model.add_adapter_fusion(["adapter1", "adapter2"], name="custom_name_fusion")

Afterwards, to address the custom-name fusion, specify the name in the Fuse block. E.g. for activation:

model.set_active_adapters(Fuse("adapter1", "adapter2", name="custom_name_fusion"))

Some fusion-specific methods can either take the named Fuse block or directly the fusion name:

# saving
model.save_adapter_fusion("./checkpoint_dir", Fuse("adapter1", "adapter2", name="custom_name_fusion"))
# or:
# model.save_adapter_fusion("./checkpoint_dir", "custom_name_fusion")

# deleting
model.delete_adapter_fusion(Fuse("adapter1", "adapter2", name="custom_name_fusion"))
# or:
# model.delete_adapter_fusion("custom_name_fusion")

[TimoImhof]

Looks good to me!