Trainer parser error: argument --fast_dev_run: invalid int value: 'True'
🐛 Bug

When generating a parser for Trainer arguments with Trainer.add_argparse_args, the type of fast_dev_run is no longer interpreted correctly (was working correctly on PL 1.08, but no longer works with PL 1.1).
When interpreting an argument like follows:
python my_script.py --fast_dev_run=True [...]
now raises the following error:
error: argument --fast_dev_run: invalid int value: 'True'
I assume this regression was introduced when changing the type of fast_dev_run from bool to Union[int, bool] as per #4629, which messes up type inference when building the parser.
Here is a Colab Notebook reproducing the issue.
Expected behavior
Even though fast_dev_run now can be an int, the automatically generated trainer CLI should continue supporting the boolean options.
Environment
Lightning 1.1