Added missing space in multi-line warning strings (#8654)

* Corrected 3 small typos in training.py error messages.

* Added missing space in multi-line warning strings

Author: mjacquem1

keras/preprocessing/image.py

@@ -536,15 +536,15 @@ def standardize(self, x):
                 x -= self.mean
             else:
                 warnings.warn('This ImageDataGenerator specifies '
-                              '`featurewise_center`, but it hasn\'t'
+                              '`featurewise_center`, but it hasn\'t '
                               'been fit on any training data. Fit it '
                               'first by calling `.fit(numpy_data)`.')
         if self.featurewise_std_normalization:
             if self.std is not None:
                 x /= (self.std + 1e-7)
             else:
                 warnings.warn('This ImageDataGenerator specifies '
-                              '`featurewise_std_normalization`, but it hasn\'t'
+                              '`featurewise_std_normalization`, but it hasn\'t '
                               'been fit on any training data. Fit it '
                               'first by calling `.fit(numpy_data)`.')
         if self.zca_whitening:
@@ -554,7 +554,7 @@ def standardize(self, x):
                 x = np.reshape(whitex, x.shape)
             else:
                 warnings.warn('This ImageDataGenerator specifies '
-                              '`zca_whitening`, but it hasn\'t'
+                              '`zca_whitening`, but it hasn\'t '
                               'been fit on any training data. Fit it '
                               'first by calling `.fit(numpy_data)`.')
         return x