set MKL_NUM_THREADS (PaddlePaddle#75880)

* MKL_NUM_THREADS

* fix

* str(os.cpu_count())

* fix2

Author: zhengshengning

python/paddle/base/__init__.py

@@ -167,6 +167,9 @@ def __bootstrap__():
     if os.getenv('NVIDIA_TF32_OVERRIDE', None) is None:
         os.environ['NVIDIA_TF32_OVERRIDE'] = '0'
 
+    if os.getenv('MKL_NUM_THREADS', None) is None:
+        os.environ['MKL_NUM_THREADS'] = str(int(0.8 * os.cpu_count()))
+
     flag_prefix = "FLAGS_"
     read_env_flags = [
         key[len(flag_prefix) :]