Add adapter_to() method for moving & converting adapter weights

[calpt]

This PR:

• introduces new methods adapter_to() & adapter_fusion_to() to move only adapter weights to device or convert dtype
• avoids moving full model in AdapterTrainer when loading best model.

Fixes #694.

[TimoImhof]

LGTM

[weiiiii0622]

Hello, i am running with the following code:

model = AutoAdapterModel.from_pretrained(f"{cache_dir}/Llama-2-7b-chat-hf/",cache_dir="checkpoints/hf_model")
model.half()
model.to("cuda")
model.set_active_adapters(None)
current_adapter = model.load_adapter(f"{adapter_dir}/baseline_adapter", config="scaled_parallel", set_active=True, load_as=f"baseline_adapter")
model.set_active_adapters(current_adapter)
# Move adapter to cuda
model.adapter_to(name=current_adapter, device="cuda:0", dtype=torch.float16)

and got the following error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[9], [line 47](vscode-notebook-cell:?execution_count=9&line=47)
     [44](vscode-notebook-cell:?execution_count=9&line=44) model_llama2.set_active_adapters(current_adapter)
     [46](vscode-notebook-cell:?execution_count=9&line=46) # Move adapter to cuda
---> [47](vscode-notebook-cell:?execution_count=9&line=47) model_llama2.adapter_to(name=current_adapter, device="cuda:0", dtype=torch.float16)
     [49](vscode-notebook-cell:?execution_count=9&line=49) # Check summary
     [50](vscode-notebook-cell:?execution_count=9&line=50) summary = model_llama2.adapter_summary()

File ~/.local/lib/python3.10/site-packages/torch/nn/modules/module.py:1614, in Module.__getattr__(self, name)
   [1612](https://vscode-remote+ssh-002dremote-002btwcc.vscode-resource.vscode-cdn.net/home/b10902138/2024.07/check-adapter-training/~/.local/lib/python3.10/site-packages/torch/nn/modules/module.py:1612)     if name in modules:
   [1613](https://vscode-remote+ssh-002dremote-002btwcc.vscode-resource.vscode-cdn.net/home/b10902138/2024.07/check-adapter-training/~/.local/lib/python3.10/site-packages/torch/nn/modules/module.py:1613)         return modules[name]
-> [1614](https://vscode-remote+ssh-002dremote-002btwcc.vscode-resource.vscode-cdn.net/home/b10902138/2024.07/check-adapter-training/~/.local/lib/python3.10/site-packages/torch/nn/modules/module.py:1614) raise AttributeError("'{}' object has no attribute '{}'".format(
   [1615](https://vscode-remote+ssh-002dremote-002btwcc.vscode-resource.vscode-cdn.net/home/b10902138/2024.07/check-adapter-training/~/.local/lib/python3.10/site-packages/torch/nn/modules/module.py:1615)     type(self).__name__, name))

AttributeError: 'LlamaAdapterModel' object has no attribute 'adapter_to'

I believe there is something wrong for LlamaAdapterModel to not have the method adapter_to.
Thanks!