Raise MisconfigurationException when the accelerator is available but…

[kaushikb11]

… the user passes invalid ([]/0/'0') values to the devices flag

What does this PR do?

Without the updates in this PR and when the accelerator is available, the user gets the following error for

Trainer(accelerator="gpu", devices=0/"0"/[])

    self._parallel_devices = self.accelerator.get_parallel_devices(self._devices_flag)
  File "/home/jovyan/pytorch-lightning/pytorch_lightning/accelerators/gpu.py", line 82, in get_parallel_devices
    return [torch.device("cuda", i) for i in devices]
TypeError: 'NoneType' object is not iterable

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cc @Borda @tchaton @rohitgr7 @akihironitta @justusschock

[justusschock]

just one very nit question :)