add cpu device count (#4484)

* add cpu device count

* import multiprocessing

dygraph/ptb_lm/ptb_dy.py

@@ -24,6 +24,7 @@ from paddle.fluid.optimizer import SGDOptimizer
 from paddle.fluid.dygraph.base import to_variable
 import numpy as np
 import six
+import multiprocessing
 
 import reader
 import model_check
@@ -217,10 +218,12 @@ def train_ptb_lm():
     model_check.check_cuda(args.use_gpu)
 
     place = core.CPUPlace()
-    if args.use_gpu == True:
-        place = core.CUDAPlace(0)
-
+    if args.use_gpu:
+        place = fluid.CUDAPlace(0)
         dev_count = fluid.core.get_cuda_device_count()
+    else:
+        place = fluid.CPUPlace()
+        dev_count = int(os.environ.get('CPU_NUM', multiprocessing.cpu_count()))
     
     # check if paddlepaddle version is satisfied
     model_check.check_version()