add warning info for CPU_NUM (#18840)

test=develop

python/paddle/fluid/framework.py

@@ -85,11 +85,14 @@ def _current_expected_place():
 
 def _cpu_num():
     if "CPU_NUM" not in os.environ.keys():
+        if multiprocessing.cpu_count() > 1:
             sys.stderr.write(
-            'The CPU_NUM is not specified, you should set CPU_NUM in '
-            'the environment variable list, i.e export CPU_NUM=1. CPU_NUM '
-            'indicates that how many CPUPlace are used in the current task.\n'
-            '!!! The default number of CPUPlaces is 1.\n\n')
+                '!!! The CPU_NUM is not specified, you should set CPU_NUM in the environment variable list.\n'
+                'CPU_NUM indicates that how many CPUPlace are used in the current task.\n'
+                'And if this parameter are set as N (equal to the number of physical CPU core) the program may be faster.\n\n'
+                'export CPU_NUM={} # for example, set CPU_NUM as number of physical CPU core which is {}.\n\n'
+                '!!! The default number of CPU_NUM=1.\n'.format(
+                    multiprocessing.cpu_count(), multiprocessing.cpu_count()))
         os.environ['CPU_NUM'] = str(1)
     cpu_num = os.environ.get('CPU_NUM')
     return int(cpu_num)