Change llama_multiprocess to multithread and execute the exp of silu to report an error.

It seems that tensor is dropped, and others are fine.

When run this line code
[candle-examples/examples/llama_multithread/model.rs#L177](https://github.com/mokeyish/candle/blob/llam_multithread/candle-examples/examples/llama_multithread/model.rs#L177)

it throws an error as following:
```txt
Cuda(Load {
cuda: DriverError(CUDA_ERROR_ILLEGAL_ADDRESS, "an illegal memory access was encountered"),
module_name: "uexp_bf16"
})
```

The reproduce code [candle-examples/examples/llama_multithread/main.rs](https://github.com/mokeyish/candle/blob/llam_multithread/candle-examples/examples/llama_multithread/main.rs).

Start with candle-examples/examples/llama_multithread/main.rs to run the reproduction, then inspect candle-examples/examples/llama_multithread/model.rs around line 177. Trace the multiprocess-to-multithread execution and the silu exp operation; done means the reproduction no longer reports CUDA_ERROR_ILLEGAL_ADDRESS at uexp_bf16.

Written by the indexing model from the issue text.