Created by: shippingwang

The concept: place confuse when someone tries to set available gpu places by indicating CUDA_VISIBLE_DEVICES

using Fleet interface, only the FLAGS_selected_gpus works

so we have to obtain gpu num by

gpu_num = paddle.fluid.core.get_cuda_device_count() if (
        'PADDLE_TRAINERS_NUM') and (
            'PADDLE_TRAINER_ID'
    ) not in env else int(env.get('PADDLE_TRAINERS_NUM', 0))

• remove this switch