Break flag bc (#3128)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3128

XNNPACK broke Backwards Compatibility by forcing pooling operations to reduce dims, and introducing a flag to allow these operation to keep the dims.

This is backwards breaking because previously XNNPACK would not keep the dims if no flag was given, now a flag must be specified to keep the dims. While initially we proposed the inverse to maintain backwards compatibility, they have encountered breakages and have decided to commit to this breakage. As we are a downstream dependency, and will have to accept this breakages ourselves, it is important that we break early before this is used in any production code.

As a result we break BC Here by accepting XNNPACK's change.

```
git diff roll_back_commit > rollback.patch
cd fbsource/fbcode/xplat/third-party/XNNPACK/XNNPACK
git apply ../../../../../../rollback.patch
```

We have to update the change in ExecuTorch, as a result we change the XNNPACK dep we are pointing to the branch containing these changes here:
https://github.com/digantdesai/XNNPACK/commits/et_v21/

Reviewed By: digantdesai

Differential Revision: D56271242

fbshipit-source-id: f05a0c98cb3b8e0b52ded9480ae5fb3ac71bbc14