pytorch

9354a68e - [codegen] split out backend-specific information from NativeFunction in the model (#57361)

[codegen] split out backend-specific information from NativeFunction in the model (#57361) Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/57361 Data model change in the codegen, which splits backend-specific information out of `NativeFunction` ### Overview Currently in the codegen, native_functions.yaml has backend-specific information about each operator that is encoded directly into the data model, in the `NativeFunction` object. That's reasonable, since the native_functions.yaml is the source of truth for information about an operator, and the data model encodes that information into types. Now that external backends can use the codegen though, that information is technically incomplete/inaccurate. In another PR, I tried patching the information on the `NativeFunction` object with the additional external information, by updating the `dispatch` entry to contain the external backend kernel name and dispatch key. Instead, this PR tries to split out that information. The `NativeFunction` class contains all information about an operator from native_functions.yaml that's backend-independent and is known never to change regardless of what extra information backends provide. We also build up a backend "index", which is basically a mapping from [backend] -> [backend-specific-metadata]. Reading in an external backend yaml just involves updating that index with the new backend. There were a few places where `NativeFunction` used the dispatch table directly, that I encoded as properties directly on the NativeFunction object (e.g. `is_abstract`). They were mostly around whether or not the operator has a composite kernel, which isn't something that's going to change for any external backends. This has a few advantages: - We can more easily re-use the existing logic in `native_function.py` and `register_dispatch_key.py` for both native and external backends, since they both involve a NativeFunction + a particular backend index - The data in the data model will be the same regardless of how the codegen is run. Running the codegen with a new external backend doesn't change the data inside of NativeFunction or an existing backend index. It just adds a new index for that backend. - There are several of codegen areas that don't care about backend-specific information: mostly the tracing and autograd codegen. We can reason about the codegen there more easily, knowing that backend-specific info is entirely uninvolved. An alternative to this split would be to augment the NativeFunction objects with external backend information at the time that we create them. So the external codegen could read both native_functions.yaml and the external backend's yaml at the same time, and construct a NativeObject with a full dispatch table (including the XLA entry), and the correct setting of structured (taking into account both yamls). One disadvantage to this approach is that NativeFunction objects now contain different stuff depending on how you ran the codegen, and you have to make sure that any changes to the codegen can properly handle all the different variants. ### Data Model Changes Removed 3 classes, which are used by the external codegen: - ExternalBackendFunction - ExternalBackendFunctionsGroup - ExternalBackendMetadata And added two new ones: - BackendIndex - BackendMetadata `BackendIndex` contains any info that's specific to that backend, plus a mapping from operator names to backend specific metadata about the operator. One example of backend-specific info that's not operator-dependent is the fact that XLA prefers to implement functional kernels instead of out kernels (and so when they eventually mark an op as structured, they're going to mark the functional op and not the out op). `BackendMetadata` contains info specific to an (operator, backend) pair. Right now, that's just (a) the name of the kernel, and (b) whether or not that operator is structured. ### Questions I wanted to get this PR up earlier so I could get feedback, but there are a few things I want to call out: **Dealing with `structured`.** This PR separates out the notion of `structured` into two bits of information: - Does [operator] have a meta() function. This is backend-agnostic, and is represented by the `structured` property on `NativeFunction`, same as before. This is used, e.g., to decide what signatures to add to `MetaFunctions.h`. - Does [operator, backend] have an impl() function. This is backend dependent; even though technically all in-tree backends are forced to write impl() functions for an operator when we port the op to structured in native_functions.yaml, out-of-tree backends can decide to opt in independently. This is represented as a property on `BackendMetadata`. This is used in most other cases, e.g. in `RegisterDispatchKey` when we're deciding whether or not to gen a structured or unstructured wrapper. I also baked `is_structured_dispatch_key` directly into each BackendIndex. So for operators marked "structured" in native_functions.yaml, their corresponding CPU/CUDA BackendIndex entries will be marked structured, and all others (except for potentially external backends) will not. I ended up trying to deal with `structured` in this change since it's technically backend dependent (XLA can opt kernels into structured separately from in-tree ops), but that may have been too ambitious: it's technically not relevant until we actually add support for structured external kernels. If it's not clear that this is the right path for dealing with structured and we want to push that off, I'm fine with backing out the bits of this PR that make `structured` backend-dependent. I don't see anything *too* controversial related to structured in the change, but I tried to call out any areas in the comments **Localizing the fact that external backends follow Dispatcher convention.** Another thing that's sort of backend specific that I didn't totally address in this PR is the fact the fact that in-tree backends follow the Native API while external backends follow the Dispatcher API. I painted over that in `native_functions.py` by adding a helper, `kernel_signature`, that takes in a native function and gives you the "correct" signature for the specified backend- NativeSignature for in-tree backends, and DispatcherSignature for out-of-tree backends. In order to make that fully useable though, we'll need `NativeSignature` and `DispatcherSignature` to have matching interfaces. I didn't bother with that in this PR, which is why `gen_external_aten_fallbacks.py` still has a bunch of direct references to the dispatcher API. Thinking of adding it in a later PR but wanted to see if anyone has other opinions. Maybe `is_external()` shouldn't even be a property on the BackendMetadata, and anything the codegen does that requires asking for that information should just be better abstracted away. **Thoughts on the `BackendIndex` / `BackendMetadata` breakdown.** One thing that's annoying right now is that to query for various pieces of metadata, you call helper functions like `backend_index.structured(f)`, which queries that particular backend and tells you if that specific NativeFunctionGroup is structured for that backend. It has to return an `Optional[bool]` though, since you have to handle the case where that operator doesn't have a kernel for that backend at all. So users of those helpers end up with a bunch of optionals that they need to unpack, even if they know at some point that the result isn't None. I think it would be easier instead to just store the NativeFunction object as a field directly on the BackendMetadata. Curious if there are any other opinions on a better way to model it though. Test Plan: Imported from OSS Reviewed By: navahgar Differential Revision: D28474362 Pulled By: bdhirsh fbshipit-source-id: 41a00821acf172467d764cb41e771e096542f661