add examples/MNIST into testing

Author: wkliao

examples/Makefile

@@ -29,9 +29,11 @@ all:
 
 check: ptest4
 	cd Pytorch_DDP && make check
+	cd MNIST && make check
 
 ptests: ptest3 ptest4 ptest8
 	cd Pytorch_DDP && make ptests
+	cd MNIST && make ptests
 
 ptest3:
 	@mkdir -p ${OUTPUT_DIR}