graph mode: add docs to pre-calibration passes (#39683)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39683
Adding a couple of docstrings to `_jit_pass_dedup_module_uses` and
`_jit_pass_insert_observers`.
Test Plan:
CI, no logic change
Imported from OSS
Differential Revision: D21975880
fbshipit-source-id: 8876e0e981d6675bce08fa8e08ac7a3d38c3c622
