[minor] torch.nimble improvements
(moved/adapted from https://github.com/fragcolor-xyz/nimtorch/issues/6#issuecomment-430484263 per request)

* I think torch.nimble could be refactored as follows (untested):
```nim
let files = ["torch/generator.nim", ..., "tests/test_xor.nim"]
task test_cuda, "Run cuda gpu tests":
  for file in files:
    exec "nim cpp -r -o:test -d:cuda " & file
    exec """nim cpp -d:release -r -o:test -d:cuda torch"""
```

* all files have nim ext in that file except `torch` => use `torch.nim` ?

* `exec """nim cpp -r -o:test torch/generator.nim"""` is repeated in several test suites identically; seems like a bug?
