[jit][static] Basic executor (#43647)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43647
Nothing fancy, just a basic implementation of the graph executor without using stack machine.
Reviewed By: bwasti
Differential Revision: D23208413
fbshipit-source-id: e483bb6ad7ba8591bbe1767e669654d82f42c356
