Complex backward for indexing, slicing, joining, and mutating ops (#49552)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49552
This PR:
1. Migrates independent autograd test for `hstack`, `dstack`, `vstack`, `movedim`, `moveaxis` from `test_autograd.py` to the new `OpInfo` based tests.
2. Migrates autograd test for `gather`, `index_select` from the method_tests to the new `OpInfo` based tests.
2. Enables complex backward for `stack, gather, index_select, index_add_` and adds tests for complex autograd for all the above mentioned ops.
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D25682511
Pulled By: anjali411
fbshipit-source-id: 5d8f89db4a9ec340ab99a6196987d44a23e2c6c6
