Add automatic mixed precision support

[egillax]

In the latest develop version of torch, automatic mixed precision has just been added. See this file and the unit test here to see how to use it

[egillax]

Here is a simple vignette describing how this works in torch:

https://torch.mlverse.org/docs/dev/articles/amp.html

[lhjohn]

Benchmark results for RTX 3090

system.time({run(autocast = FALSE, scale = FALSE)})
user  system elapsed 0.75    0.09    1.88

system.time({run(autocast = TRUE, scale = FALSE)})
user  system elapsed 0.58    0.05    1.54

system.time({run(autocast = TRUE, scale = TRUE)})
user  system elapsed 0.38    0.11    1.64