IPad is impossible, as the others have already stated.
I don't know if this is an option for you, but if portability of the device is your main concern:
spend twice the money and buy a MacbookAir 11" (i5 sufficient, but go for bigger SSD than minimal)
You would have only a small delta in weight/dimensions, but LR runs beautifully on it, really fast with the SSD...
And you can develop on it, not just rate and add other metadata.
Cornelia