expected_type argument in get_key_def() doesn't do anything

[Ling-Jun]

the expected_type in get_key_def() function may not have the intended behaviour (assume it's forcing the return value to have the expected_type),

for example: learning_rate = get_key_def(['training', 'lr'], cfg, default=None, expected_type=str) would return a float and not raise any exception. 

[CharlesAuthier]

Thanks to have reported that bug, we can see some place where it fail. We will need to clean and adjust our code.