[minor] torch.nimble improvements

[timotheecour]

(moved/adapted from #6 (comment) per request)

• I think torch.nimble could be refactored as follows (untested):

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?