Trainer.from_argparse_args fails on unknown kwargs
🐛 Bug

Since **kwargs was removed from Trainer's init in #1820, initializing Trainer objects fails if you have any non Trainer specific arguments in your parser.
If this is the expected behavior, the docs should be updated to reflect the workaround I mention below, as a few of them would currently fail.
To Reproduce
This works
parser = ArgumentParser()
parser = Trainer.add_argparse_args(parser)
args = parser.parse_args("")
trainer = Trainer.from_argparse_args(args)
Trainer init fails on unexpected kwarg 'script_specific_arg'
parser = ArgumentParser()
parser = Trainer.add_argparse_args(parser)

parser.add_argument('--script_specific_arg', type=str, default='hope this works')

args = parser.parse_args('')
trainer = Trainer.from_argparse_args(args)
Trainer init fails on unexpected kwarg 'some_argument'
class SomeModel(LightningModule):

    def __init__(self, hparams):
        super().__init__()
        self.hparams = hparams

    @staticmethod
    def add_model_specific_args(parent_parser):
        parser = ArgumentParser(parents=[parent_parser], add_help=False)
        parser.add_argument('--some_argument', type=int, default=128)
        return parser


parser = ArgumentParser()
parser = Trainer.add_argparse_args(parser)

parser = SomeModel.add_model_specific_args(parser)

args = parser.parse_args("")
trainer = Trainer.from_argparse_args(args)
You can get around it if you init Trainer on temp args
parser = ArgumentParser()
parser = Trainer.add_argparse_args(parser)

# Grab only the trainer args and init it right away
temp_args, _ = parser.parse_known_args('')
trainer = Trainer.from_argparse_args(temp_args)

parser.add_argument('--script_specific_arg', type=str, default='hope this works')
args = parser.parse_args("")
Expected behavior
Trainer.from_argparse_args should ignore unknown kwargs.
Environment

Google Colab
Current master branch (0.7.7.dev0)

Additional context

No error if using stable PyPi version (0.7.6)