Add safetensors export feature

[jonboh]

This is still a work in progress, I started out following antimora's indications in: #3260 (comment)

I did not find a clean way to walk the Module with the Serializer, and perform the serialization only with the Recorder. The problem is that once the Serializer is finished I'd need to reconstruct each TensorData back from its bytes, dtype and shape fields as the serializer will walk up to the basic types.

The alternative I found was to use the Serializer to get a mapping from ParamId to the TensorName, and use a ModuleVisitor to link to the TensorData. I think this is a bit cleaner, but the API is pretty different from the rest of the serializations, so I'm not sure, if it would be better to do it the other way, even if the logic ends up a bit more convoluted.
Any comments are appreciated :)

[antimora]

Should be optional = true and enabled via feature flag.

[antimora]

I think I know what the issue you are facing is regarding the serializer going all the way down. It's okay to stop if you come across a TensorData struct type and handle serialization differently.

My recommendation:

1. Don't use ModuleVisitor (you can get away with a serde serializer).
2. The only intermediate data structure should be a hash type of full path name + tensor safetensor view.