Lower `index_copy` (#3331)
* added index_copy to aten_xla_type and included unit tests
* dropped support for index_copy_ as discussed on the PR
* lint fix
* passed local tests, added .torch_pin, updated unit tests and very all work, remoevd index_copy_ implementation from tensor.h
* linter fix
* remove torch pin
Co-authored-by: Brian Hirsh <hirsheybar@fb.com>
