Use isinstance() instead of type() in trainer.distrib_parts.check_gpus_data_type
🐛 Bug
When instantiating a Trainer object, it makes sense to be able to pass a subclass of list.
Ideally, this would be something even more general like collections.abc.Sequence, but I'm not too familiar with Lightning's codebase and that change would have a greater likelihood of breaking things.
To Reproduce
Instantiate a Trainer with the gpus parameter being a subclass of list.
Code sample
>>> from pytorch_lightning import Trainer
>>> class MyList(list):
...     pass
... 
>>> gpus = MyList([0])
>>> t = Trainer(gpus=gpus)
This produces
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/anaconda/miniconda3/envs/ai/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 366, in __init__
    self.data_parallel_device_ids = parse_gpu_ids(self.gpus)
  File "/opt/anaconda/miniconda3/envs/ai/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 613, in parse_gpu_ids
    check_gpus_data_type(gpus)
  File "/opt/anaconda/miniconda3/envs/ai/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 561, in check_gpus_data_type
    raise MisconfigurationException("GPUs must be int, string or list of ints or None.")
pytorch_lightning.utilities.debugging.MisconfigurationException: GPUs must be int, string or list of ints or None.

Expected behavior
Trainer is instantiated normally as it would had a list been passed.
Environment

PyTorch Version: 1.4.0
PyTorch Lightning Version: 0.7.1
OS: Ubuntu 19.10
How you installed PyTorch: pip
Python version: 3.7

Potential Fix
In pytorch_lightning/trainer/distrib_parts.py check types using isinstance() instead of type():
def check_gpus_data_type(gpus):
    # if gpus is not None and type(gpus) not in (int, str, list):
    if gpus is not None and not isinstance(gpus, (int, str, list)):
        raise MisconfigurationException("GPUs must be int, string or list of ints or None.")
I'll put in a PR if this change sounds good