Add exportable baby llama example (#4345)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4345

Add a small LLaMa model, based on the babyllama paper. Note that this test case is only one layer by default, and the number of layers can be adjusted in the test.

Removed some pyre changes that broke the OSS AoT export, and added some required passes and operators.

Reviewed By: dulinriley

Differential Revision: D60073137

fbshipit-source-id: 8379296ad0aa4099b09d033b33479165d7c7c5c9