Add a `disable_dropout` utility fn

[SalmanMohammadi]

          This might be useful as a training utility, e.g. `disable_dropout(model)`

Originally posted by @ebsmothers in #2275 (comment)

[Ankur-singh]

Hi @SalmanMohammadi, I have some time this weekend. Can you please assign it to me?

[SalmanMohammadi]

Absolutely @Ankur-singh : )

[Ankur-singh]

@SalmanMohammadi and @ebsmothers what should be ideal location for function?

In torchtune/training/ directory, there are dedicated files for each functionality. disable_dropout is quite generic but there is no generic utils.py file or something similar. Any suggestions?

[acisseJZhong]

Hi @Ankur-singh you could create a model_util.py file under torchtune/training/ and put disable_dropout there. cc @ebsmothers if you have better place in mind.