Rewrite of ATen code generator (pytorch#42629)

Summary:
Pull Request resolved: pytorch#42629

How to approach reviewing this diff:

- The new codegen itself lives in `tools/codegen`. Start with `gen.py`, then read `model.py` and them the `api/` folder. The comments at the top of the files describe what is going on. The CLI interface of the new codegen is similar to the old one, but (1) it is no longer necessary to explicitly specify cwrap inputs (and now we will error if you do so) and (2) the default settings for source and install dir are much better; to the extent that if you run the codegen from the root source directory as just `python -m tools.codegen.gen`, something reasonable will happen.
- The old codegen is (nearly) entirely deleted; every Python file in `aten/src/ATen` was deleted except for `common_with_cwrap.py`, which now permanently finds its home in `tools/shared/cwrap_common.py` (previously cmake copied the file there), and `code_template.py`, which now lives in `tools/codegen/code_template.py`. We remove the copying logic for `common_with_cwrap.py`.
- All of the inputs to the old codegen are deleted.
- Build rules now have to be adjusted to not refer to files that no longer exist, and to abide by the (slightly modified) CLI.
- LegacyTHFunctions files have been generated and checked in. We expect these to be deleted as these final functions get ported to ATen. The deletion process is straightforward; just delete the functions of the ones you are porting. There are 39 more functions left to port.

Signed-off-by: Edward Z. Yang <[email protected]>

Test Plan: Imported from OSS

Reviewed By: bhosmer

Differential Revision: D23183978

Pulled By: ezyang

fbshipit-source-id: 6073ba432ad182c7284a97147b05f0574a02f763

Author: ezyang

.circleci/scripts/cpp_doc_push_script.sh

@@ -47,16 +47,11 @@ sudo apt-get -y install doxygen
 # Generate ATen files
 pushd "${pt_checkout}"
 pip install -r requirements.txt
-time python aten/src/ATen/gen.py \
+time python -m tools.codegen.gen \
   -s aten/src/ATen \
-  -d build/aten/src/ATen \
-  aten/src/ATen/Declarations.cwrap \
-  aten/src/THCUNN/generic/THCUNN.h \
-  aten/src/ATen/nn.yaml \
-  aten/src/ATen/native/native_functions.yaml
+  -d build/aten/src/ATen
 
 # Copy some required files
-cp aten/src/ATen/common_with_cwrap.py tools/shared/cwrap_common.py
 cp torch/_utils_internal.py tools/shared
 
 # Generate PyTorch files

.github/workflows/lint.yml

@@ -131,13 +131,9 @@ jobs:
             time python setup.py --cmake-only build
 
             # Generate ATen files.
-            time python aten/src/ATen/gen.py \
+            time python -m tools.codegen.gen \
               -s aten/src/ATen \
-              -d build/aten/src/ATen \
-              aten/src/ATen/Declarations.cwrap \
-              aten/src/THCUNN/generic/THCUNN.h \
-              aten/src/ATen/nn.yaml \
-              aten/src/ATen/native/native_functions.yaml
+              -d build/aten/src/ATen
 
             # Generate PyTorch files.
             time python tools/setup_helpers/generate_code.py            \