minst multi-gpu You requested GPUs: [6, 7] But your machine only has: [0, 1]
import os

import torch
from torch.nn import functional as F
from torch.utils.data import DataLoader
from torchvision.datasets import MNIST
from torchvision import transforms

import pytorch_lightning as pl


class CoolSystem(pl.LightningModule):

    def __init__(self, hparams=None):
        super().__init__()

        # get hyperparams, etc...
        self.hparams = hparams

        # not the best model...
        self.l1 = torch.nn.Linear(28 * 28, 10)

    def forward(self, x):
        # called with self(x)
        return torch.relu(self.l1(x.view(x.size(0), -1)))

    def training_step(self, batch, batch_idx):
        # REQUIRED
        x, y = batch
        y_hat = self(x)
        loss = F.cross_entropy(y_hat, y)
        tensorboard_logs = {'train_loss': loss}
        return {'loss': loss, 'log': tensorboard_logs}

    def validation_step(self, batch, batch_idx):
        # OPTIONAL
        x, y = batch
        y_hat = self(x)
        val_loss = F.cross_entropy(y_hat, y)
        return {'val_loss': val_loss}

    def validation_epoch_end(self, outputs):
        # OPTIONAL
        avg_loss = torch.stack([x['val_loss'] for x in outputs]).mean()
        tensorboard_logs = {'val_loss': avg_loss}
        return {'val_loss': avg_loss, 'log': tensorboard_logs}

    def test_step(self, batch, batch_idx):
        # OPTIONAL
        x, y = batch
        y_hat = self(x)
        return {'test_loss': F.cross_entropy(y_hat, y)}

    def test_epoch_end(self, outputs):
        avg_loss = torch.stack([x['test_loss'] for x in outputs]).mean()
        tensorboard_logs = {'test_loss': avg_loss}
        return {'test_loss': avg_loss, 'log': tensorboard_logs}

    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters(), lr=0.001)

    def prepare_data(self):
        self.mnist_train = MNIST(os.getcwd(), train=True, download=True, transform=transforms.ToTensor())
        self.mnist_test = MNIST(os.getcwd(), train=False, download=True, transform=transforms.ToTensor())

    def train_dataloader(self):
        loader = DataLoader(self.mnist_train, batch_size=32, num_workers=4)
        return loader

    def val_dataloader(self):
        loader = DataLoader(self.mnist_test, batch_size=32, num_workers=4)
        return loader

    def test_dataloader(self):
        loader = DataLoader(self.mnist_test, batch_size=32, num_workers=4)
        return loader

from pytorch_lightning import Trainer


model = CoolSystem()

# most basic trainer, uses good defaults
trainer = Trainer(gpus=[6,7],num_nodes=2,distributed_backend='ddp', progress_bar_refresh_rate=10, max_epochs=10)
trainer.fit(model)

the output is:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/multiprocessing/spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/multiprocessing/spawn.py", line 114, in _main
    prepare(preparation_data)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/multiprocessing/spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/multiprocessing/spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/runpy.py", line 263, in run_path
    pkg_name=pkg_name, script_name=fname)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home1/liuxinfang/projects/minist/model.py", line 84, in <module>
    trainer = Trainer(gpus=[6,7],num_nodes=2,distributed_backend='ddp', progress_bar_refresh_rate=10, max_epochs=10)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 438, in __init__
    self.data_parallel_device_ids = parse_gpu_ids(self.gpus)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 712, in parse_gpu_ids
    gpus = sanitize_gpu_ids(gpus)
  File "/home1/liuxinfang/anaconda3/envs/MomentRetrival/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 678, in sanitize_gpu_ids
    """)
pytorch_lightning.utilities.exceptions.MisconfigurationException: 
                You requested GPUs: [6, 7]
                But your machine only has: [0, 1]