Add `def main()` to training recipes to avoid PermissionError on Windows

[eginhard]

This is a simple issue that anyone could take on and doesn't need prior knowledge of the codebase.

Many users report errors like the following when training models on Windows (coqui-ai#1711, coqui-ai#3484, coqui-ai#1563, coqui-ai#1110, #328):

PermissionError: [WinError 32] The process cannot access the file because it is being used by another process:
'C:/Temp/TTS/TTS/recipes/ljSpeech/glow_tts/run-January-01-2024_07+21PM-5dcc16d1\trainer_0_log.txt'

This can likely be fixed by wrapping all training recipes in main() functions: https://stackoverflow.com/a/24374798

Steps:

1. For all training recipes in https://github.com/idiap/coqui-ai-TTS/tree/dev/recipes
2. Wrap the code in def main():
3. Add the end of the scripts add:

if __name__ == "__main__":
    main()