ddp only uses 1 gpu when multi gpus are available
🐛 Bug

When using accelerator=ddp and gpus=2 only one gpu is used but 2 processes are launched.
To Reproduce
CUDA_VISIBLE_DEVICES=0,1 python3 pl_examples/basic_examples/simple_image_classifier.py --accelerator ddp  --gpus 2
Here a screenshot
| NVIDIA-SMI 440.33.01    Driver Version: 440.33.01    CUDA Version: 10.2     |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  GeForce GTX 1080    Off  | 00000000:01:00.0 Off |                  N/A |
| 31%   60C    P0    51W / 240W |    512MiB /  8119MiB |     14%      Default |
+-------------------------------+----------------------+----------------------+
|   1  GeForce GTX 108...  Off  | 00000000:02:00.0 Off |                  N/A |
|  0%   50C    P2    74W / 280W |   1354MiB / 11178MiB |     80%      Default |
+-------------------------------+----------------------+----------------------+
                                                                               
+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID   Type   Process name                             Usage      |
|=============================================================================|
|    0      1866      G   ...AAAAAAAAAAAACAAAAAAAAAA= --shared-files   120MiB |
|    0      2442      G   /usr/lib/xorg/Xorg                           166MiB |
|    0      2577      G   /usr/bin/gnome-shell                         212MiB |
|    0      3464      G   /usr/lib/firefox/firefox                       1MiB |
|    0      6887      G   /usr/lib/firefox/firefox                       1MiB |
|    0     13528      G   /usr/lib/firefox/firefox                       1MiB |
|    1     20965      C   python3                                      671MiB |
|    1     20988      C   .../projects/byol-pytorch/VENV/bin/python3   671MiB |
+-----------------------------------------------------------------------------+

The two processes are 20965 and 20988 and both are using gpu 1