Support Trainer.add_argparse_args for ArgumentGroup
🚀 Feature
Support Trainer.add_argparse_args for argparse._ArgumentGroup
Motivation
Adding all of the Trainers args to an existing parser clutters its help message with many arguments. Ideally, I would like to have these in a separate argument group.
Pitch

Currently:
parser = argparse.ArgumentParser()
parser.add_argument("--my_arg")
parser = pl.Trainer.add_argparse_args(parser)
parser.print_help()
usage: script.py [-h] [--my_arg MY_ARG] [--logger [LOGGER]] [--checkpoint_callback [CHECKPOINT_CALLBACK]] [--early_stop_callback [EARLY_STOP_CALLBACK]] [--default_root_dir DEFAULT_ROOT_DIR] [--gradient_clip_val GRADIENT_CLIP_VAL] (... and many more)

optional arguments:
  -h, --help            show this help message and exit
  --my_arg MY_ARG
  --logger [LOGGER]     autogenerated by pl.Trainer
  --checkpoint_callback [CHECKPOINT_CALLBACK]
                        autogenerated by pl.Trainer
  --early_stop_callback [EARLY_STOP_CALLBACK]
                        autogenerated by pl.Trainer
  --default_root_dir DEFAULT_ROOT_DIR
                        autogenerated by pl.Trainer
  --gradient_clip_val GRADIENT_CLIP_VAL
                        autogenerated by pl.Trainer
  (... and many more)

Desired:
parser = argparse.ArgumentParser()
parser.add_argument("--my_arg")
pl_parser = parser.add_argument_group(title="pytorch-lightning Trainer arguments")
pl_parser = pl.Trainer.add_argparse_args(pl_parser)
parser.print_help()
usage: script.py [-h] [--my_arg MY_ARG] [--logger [LOGGER]] [--checkpoint_callback [CHECKPOINT_CALLBACK]] [--early_stop_callback [EARLY_STOP_CALLBACK]] [--default_root_dir DEFAULT_ROOT_DIR] [--gradient_clip_val GRADIENT_CLIP_VAL] (... and many more)

optional arguments:
  -h, --help            show this help message and exit
  --my_arg MY_ARG

pytorch-lightning Trainer arguments:
  --logger [LOGGER]     autogenerated by pl.Trainer
  --checkpoint_callback [CHECKPOINT_CALLBACK]
                        autogenerated by pl.Trainer
  --early_stop_callback [EARLY_STOP_CALLBACK]
                        autogenerated by pl.Trainer
  --default_root_dir DEFAULT_ROOT_DIR
                        autogenerated by pl.Trainer
  --gradient_clip_val GRADIENT_CLIP_VAL
                        autogenerated by pl.Trainer
  (... and many more)