add memory optimization transpiler demo

[QiJune]

Please refer to #7385 to see the benchmark result.

How to make memory optimization benchmark:

FLAGS_do_memory_benchmark=true GLOG_vmodule=executor=2 GLOG_logtostderr=1 python test_meopt_fit_a_line.py
FLAGS_do_memory_benchmark=true GLOG_vmodule=executor=2 GLOG_logtostderr=1 python test_memopt_image_classification_train.py vgg
FLAGS_do_memory_benchmark=true GLOG_vmodule=executor=2 GLOG_logtostderr=1 python test_memopt_image_classification_train.py resnet

The memory usage log will be as follows:

I0111 15:00:04.795303 2497127360 executor.cc:121] Memory used after operator adam running: 666177536
I0111 15:00:04.795395 2497127360 executor.cc:121] Memory used after operator adam running: 666177536
I0111 15:00:04.796185 2497127360 executor.cc:121] Memory used after operator adam running: 666177536
I0111 15:00:04.796331 2497127360 executor.cc:121] Memory used after operator scale running: 666177536
I0111 15:00:04.796389 2497127360 executor.cc:121] Memory used after operator scale running: 666177536
I0111 15:00:04.796910 2497127360 executor.cc:121] Memory used after operator top_k running: 666181632
I0111 15:00:04.797045 2497127360 executor.cc:121] Memory used after operator accuracy running: 666189824
I0111 15:00:04.797137 2497127360 executor.cc:121] Memory used after operator cast running: 666193920
I0111 15:00:04.797209 2497127360 executor.cc:121] Memory used after operator cast running: 666193920
I0111 15:00:04.797315 2497127360 executor.cc:121] Memory used after operator sum running: 666193920
I0111 15:00:04.848588 2497127360 executor.cc:121] Memory used after operator sum running: 666193920
I0111 15:00:04.848660 2497127360 executor.cc:121] Memory used after operator fetch running: 666198016
I0111 15:00:04.848696 2497127360 executor.cc:121] Memory used after operator fetch running: 666202112
I0111 15:00:04.918810 2497127360 executor.cc:138] -------------------------------------------------------
I0111 15:00:04.918910 2497127360 executor.cc:139] Memory used after deleting local scope: 9048064
I0111 15:00:04.918939 2497127360 executor.cc:141] -------------------------------------------------------

[reyoung]

We can just add this as a gflags, not a macro.

[QiJune]

Done

[dzhwinter]

LGTM. Thanks for such a great enhance!