!2059 fix SubsetSampler error

Merge pull request !2059 from yanghaitao/yht_fix_subset_samplers

mindspore/dataset/engine/samplers.py

@@ -506,6 +506,9 @@ class SubsetRandomSampler(BuiltinSampler):
 
     def get_num_samples(self):
         num_samples = super().get_num_samples()
+        if num_samples is None:
+            return len(self.indices)
+
         return min(len(self.indices), num_samples)