Set _remote() function args and kwargs as optional

[justinwyang]

Fixes #4300.

[robertnishihara]

Can you augment this test so that it creates an actor and calls an actor method without passing in any values for args and kwargs just to make sure that it works?

ray/python/ray/tests/test_basic.py

Line 830 in 4c80177

def test_submit_api(shutdown_only):

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[robertnishihara]

This looks great! I left a few comments. It looks like there is a pre-existing bug in the test that you've uncovered.

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[robertnishihara]

@justinwyang I pushed some minor linting changes, this looks great! I'll merge this once the tests pass.

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