torch.jit.script_if_tracing
torch.jit.script_if_tracing(fn)
Compiles "fn" when it is first called during tracing.
   "torch.jit.script" has a non-negligible start up time when it is
   first called due to lazy-initializations of many compiler builtins.
   Therefore you should not use it in library code. However, you may
   want to have parts of your library work in tracing even if they use
   control flow. In these cases, you should use
   "@torch.jit.script_if_tracing" to substitute for
   "torch.jit.script".
Parameters:
      fn -- A function to compile.
Returns:
      If called during tracing, a "ScriptFunction" created by
      torch.jit.script is returned. Otherwise, the original function
      fn is returned.