Non-Configurable GPU Count via Arguments

[willyfh]

In this code, https://github.com/microsoft/UniVL/blob/main/main_task_caption.py#L143-L145 , the value of n_gpu is determined by the actual number of GPUs present in the system. However, there are scenarios where utilizing only a single GPU, despite having multiple GPUs available, could be preferable. Rather than forcibly adjusting the args.n_gpu based solely on the detected number of GPUs, it could be beneficial to leave this decision to the user.