I want to apply for an internship in Mila Group and work with of the professors. For internship, I should fill the internship form and prepare a letter of motivation. The founder of the group is Prof. Bengio, but I want to apply to work with someone else. Now, I don't know in the start of motivation letter, the receiver should be the professor I want to work with or Prof. Bengio or someone else? If the receiver is Prof. Bengio, how I should state the other one?
If the application does not specify who to address, simply "Dear Sirs/Madams," "Dear members of selection committee," etc. would suffice.
Do not address it to the Director unless you're asked to, it'd look too presumptuous.
Now in the motivation letter, you may make sensible association between your interest and some professors' work. I'd recommend wording it with a bit more openness and flexibility. Sometimes that professor may be already fully committed to other students, sometimes the committee may deem another professor a better match for you. You don't usually want to make it so firm that "it's him/her or I am not interested."
Lastly, it's not a weakness to ask questions. If any of the above still makes you feeling uneasy, send the admission person at Mila an e-mail.