Join Multiple Tensors into One Tensor

## Arguments

- tensors
list of

`Tensor`

instances- temporary
whether to garbage collect space when exiting R session

## Value

A new `Tensor`

instance with the last dimension

## Details

Merges multiple tensors. Each tensor must share the same dimension
with the last one dimension as 1, for example, `100x100x1`

. Join 3
tensors like this will result in a `100x100x3`

tensor. This function
is handy when each sub-tensors are generated separately. However, it does no
validation test. Use with cautions.

## Examples

```
tensor1 <- Tensor$new(data = 1:9, c(3,3,1), dimnames = list(
A = 1:3, B = 1:3, C = 1
), varnames = c('A', 'B', 'C'))
tensor2 <- Tensor$new(data = 10:18, c(3,3,1), dimnames = list(
A = 1:3, B = 1:3, C = 2
), varnames = c('A', 'B', 'C'))
merged <- join_tensors(list(tensor1, tensor2))
merged$get_data()
#> , , C = 1
#>
#> B
#> A 1 2 3
#> 1 1 4 7
#> 2 2 5 8
#> 3 3 6 9
#>
#> , , C = 2
#>
#> B
#> A 1 2 3
#> 1 10 13 16
#> 2 11 14 17
#> 3 12 15 18
#>
```