use git submodules

[pytorch]

use git submodules

[dskhudia]

I think so. For now, you can download asmjit once and point ASMJIT_SRC_DIR env var to the downloaded source code to avoid downloading with build. Particular git tag we use is in https://github.com/pytorch/FBGEMM/blob/master/cmake/modules/DownloadASMJIT.cmake

[dhpollack]

@t-vi I run into this problem all the time too. I created a pull request. Perhaps you could test it out and let me know if it works for you.

[dskhudia]

@dhpollack Thanks a lot for the pull request. Let me test its impact on Caffe2/Pytorch build.

[dskhudia]

Update: I had to revert the commit because adding submodules involves one more step so that internal tools work correctly. Will re-commit soon.

[dhpollack]

@dskhudia I also noticed a few other things that I can update before you commit this pr.

1. I may want to use ${FBGEMM_SOURCE_DIR} instead of ${CMAKE_SOURCE_DIR}, because I'm not sure what the latter is when FBGEMM is a submodule itself.
2. You may want to consider renaming ${ASMJIT_SRC_DIR} to ${ASMJIT_SOURCE_DIR} to make it consistent with the other variables.

Let me know if you want me to make those changes. I am not sure it matters as both work for me right now.

[dskhudia]

1. I will change it to FBGEMM_SOURCE_DIR
2. I remember it being having an issue with ASMJIT's cmake file. So we should keep that.

[dskhudia]

@t-vi and @dhpollack : It's fixed now. Once the pytorch's fbgemm submodule gets updated (should happen soon) to fbgemm's latest commit, pytorch build will not download asmjit at build time.

@dhpollack : Thanks for your PR 👍