Disable validation when val_percent_check=0 (#1251)

* fix disable validation

* add test

* update changelog

* update docs for val_percent_check

* make "fast training" docs consistent

pytorch_lightning/trainer/trainer.py

@@ -876,7 +876,8 @@ class Trainer(
             return
 
         # check if we should run validation during training
-        self.disable_validation = not self.is_overriden('validation_step') and not self.fast_dev_run
+        self.disable_validation = not (self.is_overriden('validation_step') and self.val_percent_check > 0) \
+            and not self.fast_dev_run
 
         # run tiny validation (if validation defined)
         # to make sure program won't crash during val