[inductor] modify index_reduce to pass opinfo tests (#106429)
1. add a python meta registration, to fix an issue with the forward pass. The problem was that previously, the C++ meta registration calls [numel()](https://github.com/pytorch/pytorch/blob/7b14a14e27d90b38a0a02b93592e8e81bd9afa44/aten/src/ATen/native/TensorAdvancedIndexing.cpp#L329) which fails (LMK if it's better to fix the C++ implementation to not do this check)
2. Modify the backward to fix an issue in the backward. The backward is not a custom op - it's a custom manual backward implementation. In particular, there's some situations that don't support double backward; the check for whether double backward is allowed requires a .item() call. To fix the meta/fake tensor case, this PR will avoid setting the double backward error only if `GradMode::is_enabled()` - which shouldn't be turned on in PT2.
3. Update skips.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106429
Approved by: https://github.com/zou3519
