Introduce apply_xla_patch_to_nn_linear and test that in a scan #8739
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In order to propagate sharding annotations in 2D sharding, linear layers should be implemented with einsum instead of tranposes/reshapes. Additionally, they need to continue to function inside scan/scan_layers. For this to work we need three pieces: - I added a `apply_xla_patch_to_nn_linear` function to replace the implementation of `nn.Linear` with einsum (calling XLAPatchedLinear). - The XLAPatchedLinear implementation should be wrapped in torch custom ops. That's because AOTAutograd used by scan will decompose all einsums into transposes/reshapes, unless we use `@custom_op` to mark a function as opaque to AOTAutograd. - Even after wrapping them with `@custom_op`, the einsum is still decomposed into transposes/reshapes due to #8713. That's a bug/PyTorch limitation. To workaround this, I added a `_xla_einsum` C++ function that directly builds an einsum given XLA tensors, skipping over any PyTorch dispatcher complexity. Added a test that demonstrates how `nn.Linear` layers by default flattens any non-contracting dims, and how we could avoid that with `apply_xla_patch_to_nn_linear`.
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zpcore
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In order to propagate sharding annotations in 2D sharding, linear layers should be implemented with einsum instead of tranposes/reshapes. Additionally, they need to continue to function inside scan/scan_layers.
For this to work we need three pieces:
apply_xla_patch_to_nn_linearfunction to replace the implementation ofnn.Linearwith einsum (calling XLAPatchedLinear).@custom_opto mark a function as opaque to AOTAutograd.@custom_op, the einsum is still decomposed into transposes/reshapes due totorch.einsum is incorrectly decomposed when wrapped inside a custom op #8713. That's a bug/PyTorch limitation. To workaround this, I added a
_xla_einsumC++ function that directly builds an einsum given XLA tensors, skipping over any PyTorch dispatcher complexity.Added a test that demonstrates how
nn.Linearlayers by default flattens any non-contracting dims, and how we could avoid that withapply_xla_patch_to_nn_linear.