Sgn implementation (#3045)
* added the code skeleton
added BuildSgn, added test, all tests pass
added test_sgn() to test_operations.py
linter fix
reverted the linter change to test_ops
fixed python format
* addressed test review comments
* format fix
* more tests to guarantee -inf and inf
small fix
added debug prints
format fix
replaced sign with sgn call in aten_xla_type
* added inf/nan handling in BuildSgn
* format fix, cleanup
* improving data type support for sgn
* improving data type support for sgn
* improving data type support for sgn
* addressed review comments, removed debug code
* format fix
* resubmitting the sgn() operation test logic changes
resubmitting the sgn() operation test logic changes
