Change max to abs for preprocess time (#240)

`normal` result can be negative.
if we do `max(t, 0) then we will reduce the variance when the generated value is negative.
It is better to use `abs` in this case.

dlio_benchmark/reader/reader_handler.py

@@ -60,9 +60,9 @@ def __init__(self, dataset_type, thread_index):
 
     @dlp.log
     def preprocess(self, a=None):
-        if self._args.preprocess_time != 0. or self._args.preprocess_time_stdev != 0.:
+        if self._args.preprocess_time > 0. or self._args.preprocess_time_stdev > 0.:
             t = np.random.normal(self._args.preprocess_time, self._args.preprocess_time_stdev)
-            sleep(max(t, 0.0))
+            sleep(abs(t))
         return a
     @abstractmethod
     def open(self, filename):
@@ -151,4 +151,4 @@ def is_index_based(self):
 
     @abstractmethod
     def is_iterator_based(self):
-        return False
+        return False