[caffe2] Fix invalid vector accesses and polar() call (#66757)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66757
`InterpreterStateImpl::run()` gets the number of outputs from the current frame, but by the time the continuation completes, the frame is gone, so we're calling `front()` on an empty vector. This works out in practice (data is still there) but it is technically undefined behavior and could break in the future.
Also, `std::polar()` expects its argument to be non-negative, but `c10::polar()` does not, so implement it explicitly (implementation is the same as libstdc++).
Test Plan: JIT tests pass.
Reviewed By: zhxchen17
Differential Revision: D31715587
fbshipit-source-id: 98abcc10c2742887af866d8e70169a0187c41d33
