[tensorexpr] Autograd for testing (#42548)
Summary:
A simple differentiable abstraction to allow testing of full training graphs.
Included in this 1st PR is an example of trivial differentiation.
If approved, I can add a full MLP and demonstrate convergence using purely NNC (for performance testing) in the next PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42548
Reviewed By: ZolotukhinM
Differential Revision: D23057920
Pulled By: bwasti
fbshipit-source-id: 4a239852c5479bf6bd20094c6c35f066a81a832e
