Decoder native functions join the dead code society (#96025)
Summary: Decoder native joins the dead code society
With the recent introduction of PT2, we no longer need native decoder operators:
1 - full-function SDPA kernels can be used to implement cross-attention efficiently without the (slower) decoder MHA blob.
2 - torch.compile() generates more efficient code across many platforms from the python implementation of decoders than the decoder layer blob by tailoring code to target
Test Plan: github & sandcastle
Differential Revision: D43811808
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96025
Approved by: https://github.com/ezyang, https://github.com/albanD
