Fix merge of boolean experiment variables with string from CLI

[wico-silva]

This PR fixes boolean conversion from CLI strings when merging with experiment configuration.

This is the current behavior:

python tools/train.py ... save_history_ckpt False

results in self.save_history_ckpt=True because the function merge() executes the following code:

v = src_type(v) -> self.save_history_ckpt = bool("False") -> self.save_history_ckpt = True

In Python, any non-empty string when cast to bool returns True.

Naturally, the expected behavior is actually self.save_history_ckpt = False.

[FateScript]

waht if you try save_history_ckpt "True"

[wico-silva]

Without this PR:

• save_history_ckpt "True" will result in self.save_history_ckpt = True, as expected.
• Not expected is that save_history_ckpt "False" also results in self.save_history_ckpt = True.
• Any character/string inside the quotes will result in self.save_history_ckpt = True.
• The only way to make it False is with save_history_ckpt "".

With this PR:

• save_history_ckpt "True" will result in self.save_history_ckpt = True.
• save_history_ckpt "False" will result in self.save_history_ckpt = False.