Explicit disposal discussion

[dsyme]

TorchSharp is optionally allowing explicit disposal of TorchSharp handles to tensors. Currently DiffSharp doesn't support this - you can't explicitly dispose DiffSharp tensors. We should consider if DiffSharp is also going to allow explicit disposal somehow.

Explicit disposal brings accuracy with regard to memory/performance but is quite intrusive in the programming model. For example, TorchSharp is always guaranteeing that new handles get returned from tensor functions, so

let someFunction (input: Tensor) = 
    if monday then
       input.add(100)
    else
      input

must become:

let someFunction (input: Tensor) = 
    if monday then
       input.add(100)
    else
      input.alias() // create a new handle, because tensor functions always return new handles that can be explicitly disposed.

This means tensor functions creating intermediaries can explicitly dispose, e.g. this is valid (but absolutely relies on f1 returning a new tensor handle - not necessarily a copy of the tensor, but certainly a fresh alias handle).

let compose f1 f2 x =
    use tmp = f1 x
    f2 tmp

Now TorchSharp doesn't have much compositional second-order code (objects composing functions) - th Sequential model is the most obvious example. However DiffSharp has a lot of this. Overall the above will be very intrusive for DiffSharp programming. It's a bit of a conundrum to be honest.