Built-in support for saving/loading model weights in safetensors format

[minghuaw]

We already have support for conversion between mlx_rs::Array and safetensors::tensor::TensorView, so supporting this wouldn't be hard. However, there might be an asymmetry in the saving/loading API due to the lack of public API to create an SafeTensors from Array. More specifically, the only public API to create SafeTensors is SafeTensors::deserialize(buf), where buf is &[u8].

This could end up with some API look like below

fn load_safetensors(model: &mut impl ModuleParameters, safetensors: SafeTensors<'_>) -> Result<()> { }

fn save_safetensors(model: & impl ModuleParameters, path: impl AsRef<Path>) -> Result<()> { }

where we have an asymmetry that we can only save to a file rather than a SafeTensors object.

Or we could do something similar to candle-nn where both loading and saving take a Path to a safetensors file

[minghuaw]

#178 provides a partial fulfillment of this feature, and both saving and loading would be dealing with a impl AsRef<Path>. The parts that is missing in #178 are

• Performance of loading weights. Loading weights in the mistral example is two times slower than the original python example
• Support loading/saving from/to multiple files