[pytorch] reintroduce static dispatch (#51957)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51957
This is a simplified version of #51554.
Compared to #51554, this version only supports statically dispatching to
a specific backend. The benefit is that it skipped the dispatch key
computation logic thus has less framework overhead. The downside is that
if input tensors do not match the specified backend it will throw error
instead of falling back to regular dispatch.
Sample code:
```
Tensor empty(IntArrayRef size, TensorOptions options, c10::optional<MemoryFormat> memory_format) {
return at::cpu::empty(size, options, memory_format);
}
// aten::conj(Tensor(a) self) -> Tensor(a)
Tensor conj(const Tensor & self) {
return at::math::conj(self);
}
// aten::conj.out(Tensor self, *, Tensor(a!) out) -> Tensor(a!)
Tensor & conj_out(Tensor & out, const Tensor & self) {
return at::cpu::conj_out(out, self);
}
// aten::conj.out(Tensor self, *, Tensor(a!) out) -> Tensor(a!)
Tensor & conj_outf(const Tensor & self, Tensor & out) {
return at::cpu::conj_out(out, self);
}
// aten::_conj(Tensor self) -> Tensor
Tensor _conj(const Tensor & self) {
return at::defaultbackend::_conj(self);
}
```
For ops without the specific backend dispatch, it will throw error:
```
// aten::_use_cudnn_ctc_loss(Tensor log_probs, Tensor targets, int[] input_lengths, int[] target_lengths, int blank) -> bool
bool _use_cudnn_ctc_loss(const Tensor & log_probs, const Tensor & targets, IntArrayRef input_lengths, IntArrayRef target_lengths, int64_t blank) {
TORCH_CHECK(false, "Static dispatch does not support _use_cudnn_ctc_loss for CPU.");
}
```
Differential Revision: D26337857
Test Plan: Imported from OSS
Reviewed By: bhosmer
Pulled By: ljk53
fbshipit-source-id: a8e95799115c349de3c09f04a26b01d21a679364
