Support Mixtral on macOS (#1558)

A follow-up of my previous PR (#1529).

This PR makes Mixtral work on Metal GPUs that macOS comes with. There
are honestly no much change needed, except for that Metal doesn't
support fp64 data types.

A python script to run Mixtral:

```python
from mlc_chat import ChatConfig, ChatModule, callback
from mlc_chat.support import logging
logging.enable_logging()

MODEL = "HF://junrushao/Mixtral-8x7B-Instruct-v0.1-q4f16_1-MLC"
NUM_GPU = 1

def main():
    cm = ChatModule(MODEL, chat_config=ChatConfig(
        sliding_window_size=1024,
        tensor_parallel_shards=NUM_GPU,
    ))
    cm.generate("What is the meaning of life?", progress_callback=callback.StreamToStdout(callback_interval=2))

if __name__ == "__main__":
    main()
```

Quantization formats:
- 3-bit (19.662 GB): ["HF://junrushao/Mixtral-8x7B-Instruct-v0.1-q3f16_1-MLC"](https://huggingface.co/junrushao/Mixtral-8x7B-Instruct-v0.1-q3f16_1-MLC)
- 4-bit (24.466 GB): ["HF://junrushao/Mixtral-8x7B-Instruct-v0.1-q4f16_1-MLC"](https://huggingface.co/junrushao/Mixtral-8x7B-Instruct-v0.1-q4f16_1-MLC)

Author: junrushao

python/mlc_chat/op/moe_misc.py

@@ -186,7 +186,7 @@ def moe_cumsum(expert_indices: Tensor, num_local_experts: int) -> Tensor:
         .permute_dims(1, 0)
         .reshape(batch_size * num_local_experts)
     )
-    with Target(
+    with Target.current(allow_none=True) or Target(
         {
             "kind": "cuda",
             "max_num_threads": 1024,