Is there a way to dedicate the discrete graphic card to CUDA on a MacBook Pro?
The solution for that kind of problem is to shut down the graphical windows management and to use Mac OS from a TTY. This is cumbersome, but not as much as to dual-boot your machine and to switch from one OS to the other.

The way to do that is to go into Settings->Users and to set the login options to "username and passwords" (default is user list). 
Then log out all users and go to log screen

In the login testbox, fill in &gt;console
and type your password in the password box (you must have admin rights, but I assume this is already the case)

Here you go, you're in text mode and the GPU is fully available for CUDA. You gotta know the basics of the shell though...