Add gpus argument for chainerrl.misc.set_random_seed

[muupan]

• Add gpus argument to chainerrl.misc.set_random_seed to set a seed of cupy.random as well as random and numpy.random.
• Add docstring and tests

[muupan]

The test passed with cupy and a single gpu.

I also manually checked with two gpus:

$ python -c "import cupy; import chainerrl; chainerrl.misc.set_random_seed(0, gpus=(0,1)); cupy.cuda.Device(0).use(); print(cupy.random.rand())"
0.5615491552815766
$ python -c "import cupy; import chainerrl; chainerrl.misc.set_random_seed(0, gpus=(0,1)); cupy.cuda.Device(1).use(); print(cupy.random.rand())"
0.5615491552815766
$ python -c "import cupy; import chainerrl; chainerrl.misc.set_random_seed(1, gpus=(0,1)); cupy.cuda.Device(0).use(); print(cupy.random.rand())"
0.27550817398125504
$ python -c "import cupy; import chainerrl; chainerrl.misc.set_random_seed(1, gpus=(0,1)); cupy.cuda.Device(1).use(); print(cupy.random.rand())"
0.27550817398125504

[toslunar]

LGTM.

Let me leave a survey on seeds.

• random.seed (with the current default version=2) uses all the bits of int.
• numpy.random.seed accepts int in [0, 2 ** 32).
• cupy.random.seed accepts int that can be casted safely to uint64.