Provide a `clone_to_device` method for `Module`

[edlanglois]

I'd like to copy a module to another device. In C++ this is the clone method with an optional device.

While not something I currently need, I think it would also make sense to implement Clone for Module where it does a deep copy to the same device (and the same for implementing Clone for Tensor, but maybe that's a separate discussion).

Anyways, for C++ it looks like they have a separate Cloneable template class so a separate trait for clone_to_device would be an option rather than implementing directly on Module.

I'd be happy to try making a PR for this.

[edlanglois]

I've discovered the strategy you use in the DDPG example of constructing a new module instance then using the var store to copy the variables. That should work for me.

Having looked at the C++ implementation of Cloneable, doing something analogous would require expanding the Module trait to include parameters, buffers, and children. Alternatively it could be something implemented on a per-Module basis using Tensor::to / hypothetical Tensor::clone.

Those are bigger changes than I'd want to make given the existence of the VarStore approach so I'm no longer planning on making a PR for this at the moment.

[jerry73204]

You can see my tch-tensor-like macro that derives .shallow_clone() on compound tensor structs.

[LaurentMazare]

Closing this for now as it has been a while and it feels like support this via some external crate such as tch-tensor-like is probably the best.