When I left for work this morning, my dog was in the backyard. There was a little bit of rain, but he's a Labrador-type dog, and the rain doesn't usually bother him much.
After I left, it started to really pour, in sheets. My wife didn't realize he was in the backyard, because he didn't bark to get let in - something he is perfectly capable of doing. When she did let him in, he had had a thorough shower.
As a rule, you don't have to train
a Labrador to tolerate rain.