Repository restructuring

[lockshaw]

Description of changes:

Related Issues:

Linked Issues:

• Issue #

Issues closed by this PR:

• Closes #

Before merging:

• Did you update the flexflow-third-party repo, if modifying any of the Cmake files, the build configs, or the submodules?

[lockshaw]

@virena FYI

[eric-zheng]

Great to see this refactor of the repo structure which also cleans up old code. It's also good to see that we are going to use spdlog for logging. Just a note that from my past experience, spdlog may significantly increase the compilation time (not necessarily in ff though).

[lockshaw]

Great to see this refactor of the repo structure which also cleans up old code. It's also good to see that we are going to use spdlog for logging. Just a note that from my past experience, spdlog may significantly increase the compilation time (not necessarily in ff though).

Any reason why? We're not using the header-only version so it should only even contribute on the first build

[reyna-abhyankar]

Closes #440 (element unary kernel refactor)

[lockshaw]

Moving to #622