update worker keys and add test

[andrewkho]

Users [internal] are occasionally using json.dumps to serialize the state_dict when they know the state_dicts are JSON serializable. Ignoring the fact this won't work in general, this fails because json spec does not allow int properties (string required) which we're using for _worker_states in the state dict. This PR changes the format of the state_dict to use strings instead, ie _worker_states: {0: ..., 1: ...} -> _worker_states: {"worker_0": ..., "worker_1": ...}

Changes

• Adds json serde test which fails before fix

• Sets worker state keys to explicitly be strings for zero ambiguity

[facebook-github-bot]

@andrewkho has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@andrewkho merged this pull request in pytorch/data@c1f8b66f9b6e87625541f3f915c7822245d609b8.