Fix in file position logic: file descriptor and Python-side handle (#20270)
Summary:
This addresses #18436
The logic replicates the essence of closing file descriptors in numpy:
https://github.com/numpy/numpy/blob/bf20e3034085716c4559ec4bf31b23b6016f266c/numpy/core/include/numpy/npy_3kcompat.h#L278
This stores the position of the file descriptor before resetting it to the Python handle offset, then resets to the original position before exit. The Python-side handle is then updated to reflect the new position. Also added somewhat more demanding tests to cover this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20270
Differential Revision: D15275902
Pulled By: soumith
fbshipit-source-id: 5ca8a52b61c7718d2e69571f72f80b1350b0acdb
