Add the ability to specify the number of GPUs

[jan-janssen]

No description provided.

[jan-janssen]

Simple example code:

from pympipool import Executor
from tensorflow.python.client import device_lib

def get_available_gpus():
    local_device_protos = device_lib.list_local_devices()
    return [x.name for x in local_device_protos if x.device_type == 'GPU']


if __name__ == "__main__":
    with Executor(cores=1, gpus_per_task=1, enable_flux_backend=True) as exe:
        fs = exe.submit(get_available_gpus)
    
    print(fs.result())