Restructured torch modules to support shapr installation without torch

[LHBO]

In this PR, we refactor the vaeac approach so that the torch-modules are initiated through functions. Previously, when they were not inside functions, installation of shapr failed as it tried to evaluate torch::, which was not installed.
We tried to fix this in #390, but do to technical issues for me, I had to make a new PR.

Also fixed some typos in the Roxygen documentation and ensured that the progressr progress bar inside the vaeac approach is only called if progressr is available.

Details:
Instead of having

vaeac_dataset <-  torch::dataset(
  name = "vaeac_dataset", 
  initialize = function(X, one_hot_max_sizes) {...},
  .getbatch = function(index) {...},
  .length = function() {...}
 )

we replaced it with

vaeac_dataset <- function(X, one_hot_max_sizes) {
  vaeac_dataset_tmp <- torch::dataset(
    name = "vaeac_dataset", 
    initialize = function(X, one_hot_max_sizes) {...},
    .getbatch = function(index) {...},
    .length = function() {...}
  )
  return(vaeac_dataset_tmp(X = X, one_hot_max_sizes = one_hot_max_sizes))
}

Some extra care had to be given to memory_layer, which had an internal and shared environment between all instances of memory_layer. In the new version, we have to create an environment first and then send this to the new version of memory_layer.

[martinju]

I tested this locally both with and without the torch package, and it seems to work fine.

To test it installs fine without the torch package, deleted the torch package and then installed the pacakge form this branch by devtools::install_github("LHBO/shapr",ref = "Lars/torch_module_restructure"). I can then run e.g. the readme example. When doing devtools::document() I get errors that torch is not installed. This seems to be related to the reference to torch in the documentation. That is anyway fine, and similar to what we get trying to run an example involving torch.

When having torch installed, all checks pass locally, and I can run devtools::document() without errors.

I just checked the code briefly to understand conceptually what you did, but didn't inspect it in detail.
Please write a brief summary of what you did in the PR description, then you can just merge.

Well done!