Add a demo backend with compiler (#52603)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52603
This PR introduced a backend with minimum compilation capability to the to_<backend> flow. The targets are:
- Demonstrate the end-to-end flow with adding a backend -> compilation -> runtime
- How the backend compilation errors be surfaced to the user, with the original model's source code information. (C++ only in this PR. Python APIs will be demonstrated in a following PR.)
Changes:
- Compilation
1. A backend with minimum compilation features, "backend_with_compiler_demo" is added.
2. The compilation happens AOT in the ```pre_process``` function registered to this backend.
3. Compiled results are stored in a string blob for each method. They are serialized to the lowered module with ```__get_state__``` function.
4. Error message with model source code is thrown, for features not handled by the backend compiler.
- Runtime
1. The compiled blob is loaded in ```__set_state__``` method.
2. The ```compile``` function of the backend pass through the AOT compiled blob. (TODO: parsing the blob to the format that the backend can understand can happen here.)
3. The ```execute``` function of the backend executes the specified method (handle).
Test Plan:
- ```BackendTest.TestCompiler```: the C++ end-to-end demonstration on a supported model. After compilation and running, the lowered model produces the same result as the original torchscript model.
- ```BackendTest.TestCompilerNotSupport```: Demonstrate the error message from the AOT compilation for a feature not supported from the input module. The error message looks like:
```
"The node of aten::mul is not supported in this compiler. Source code: File "<string>", line 3
def forward(self, x, h):
return x * h
~~~~~ <--- HERE
```
Reviewed By: raziel
Differential Revision: D26593968
Pulled By: iseeyuan
fbshipit-source-id: 8f264f60a0470e9f07e36fdeccbf17da6c1d7cd7
